Coq Version 8.4 for the Clueless (175 Hints)

# Coq Version 8.4 for the Clueless (175 Hints) ### Pierre Castéran Hugo Herbelin Florent Kirchner Benjamin Monate Julien Narboux

> Abstract: This note intends to provide an easy way to get acquainted with the Coq theorem prover. It tries to formulate appropriate answers to some of the questions any newcomers will face, and to give pointers to other references when possible.

## Contents

## 1 Introduction

This FAQ is the sum of the questions that came to mind as we developed proofs in Coq. Since we are singularly short-minded, we wrote the answers we found on bits of papers to have them at hand whenever the situation occurs again. This is pretty much the result of that: a collection of tips one can refer to when proofs become intricate. Yes, it means we won’t take the blame for the shortcomings of this FAQ. But if you want to contribute and send in your own question and answers, feel free to write to us…

## 2 Presentation

#### 1 What is Coq?

The Coq tool is a formal proof management system: a proof done with Coq is mechanically checked by the machine. In particular, Coq allows:

- the definition of mathematical objects and programming objects,
- to state mathematical theorems and software specifications,
- to interactively develop formal proofs of these theorems,
- to check these proofs by a small certification “kernel”.

Coq is based on a logical framework called “Calculus of Inductive Constructions” extended by a modular development system for theories.

#### 2 Did you really need to name it like that?

Some French computer scientists have a tradition of naming their software as animal species: Caml, Elan, Foc or Phox are examples of this tacit convention. In French, “coq” means rooster, and it sounds like the initials of the Calculus of Constructions CoC on which it is based.

#### 3 Is Coq a theorem prover?

Coq comes with decision and semi-decision procedures ( propositional calculus, Presburger’s arithmetic, ring and field simplification, resolution, ...) but the main style for proving theorems is interactively by using LCF-style tactics.

#### 4 What are the other theorem provers?

Many other theorem provers are available for use nowadays. Isabelle, HOL, HOL Light, Lego, Nuprl, PVS are examples of provers that are fairly similar to Coq by the way they interact with the user. Other relatives of Coq are ACL2, Agda/Alfa, Twelf, Kiv, Mizar, NqThm, Omega…

#### 5 What do I have to trust when I see a proof checked by Coq?

You have to trust:

The theory behind Coq
  The theory of Coq version 8.0 is generally admitted to be consistent wrt Zermelo-Fraenkel set theory + inaccessible cardinals. Proofs of consistency of subsystems of the theory of Coq can be found in the literature.
The Coq kernel implementation
  You have to trust that the implementation of the Coq kernel mirrors the theory behind Coq. The kernel is intentionally small to limit the risk of conceptual or accidental implementation bugs.
The Objective Caml compiler
  The Coq kernel is written using the Objective Caml language but it uses only the most standard features (no object, no label ...), so that it is highly improbable that an Objective Caml bug breaks the consistency of Coq without breaking all other kinds of features of Coq or of other software compiled with Objective Caml.
Your hardware
  In theory, if your hardware does not work properly, it can accidentally be the case that False becomes provable. But it is more likely the case that the whole Coq system will be unusable. You can check your proof using different computers if you feel the need to.
Your axioms
  Your axioms must be consistent with the theory behind Coq.

#### 6 Where can I find information about the theory behind Coq?

The Calculus of Inductive Constructions
  The corresponding chapter and the chapter on modules in the Coq Reference Manual.
Type theory
  A book [11] or some lecture notes [8].
Inductive types
  Christine Paulin-Mohring’s habilitation thesis [19].
Co-Inductive types
  Eduardo Giménez’ thesis [9].
Miscellaneous
  A bibliography about Coq

#### 7 How can I use Coq to prove programs?

You can either extract a program from a proof by using the extraction mechanism or use dedicated tools, such as Why, Krakatoa, Frama-c, to prove annotated programs written in other languages.

#### 8 How old is Coq?

The first implementation is from 1985 (it was named CoC which is the acronym of the name of the logic it implemented: the Calculus of Constructions). The first official release of Coq (version 4.10) was distributed in 1989.

#### 9 What are the Coq-related tools?

There are graphical user interfaces:

Coqide
  A GTK based GUI for Coq.
Pcoq
  A GUI for Coq with proof by pointing and pretty printing.
coqwc
  A tool similar to wc to count lines in Coq files.
Proof General
  A emacs mode for Coq and many other proof assistants.
ProofWeb
  The ProofWeb online web interface for Coq (and other proof assistants), with a focus on teaching.
ProverEditor
  is an experimental Eclipse plugin with support for Coq.

There are documentation and browsing tools:

coq-tex
  A tool to insert Coq examples within .tex files.
coqdoc
  A documentation tool for Coq.
coqgraph
  A tool to generate a dependency graph from Coq sources.

There are front-ends for specific languages:

Why
  A back-end generator of verification conditions.
Krakatoa
  A Java code certification tool that uses both Coq and Why to verify the soundness of implementations with regards to the specifications.
Caduceus
  A C code certification tool that uses both Coq and Why.
Zenon
  A first-order theorem prover.
Focal
  The Focal project aims at building an environment to develop certified computer algebra libraries.
Concoqtion
  is a dependently-typed extension of Objective Caml (and of MetaOCaml) with specifications expressed and proved in Coq.
Ynot
  is an extension of Coq providing a "Hoare Type Theory" for specifying higher-order, imperative and concurrent programs.
Ott
  is a tool to translate the descriptions of the syntax and semantics of programming languages to the syntax of Coq, or of other provers.

#### 10 What are the high-level tactics of Coq

- Decision of quantifier-free Presburger’s Arithmetic
- Simplification of expressions on rings and fields
- Decision of closed systems of equations
- Semi-decision of first-order logic
- Prolog-style proof search, possibly involving equalities

#### 11 What are the main libraries available for Coq

- Basic Peano’s arithmetic, binary integer numbers, rational numbers,
- Real analysis,
- Libraries for lists, boolean, maps, floating-point numbers,
- Libraries for relations, sets and constructive algebra,
- Geometry

#### 12 What are the mathematical applications for Coq?

Coq is used for formalizing mathematical theories, for teaching, and for proving properties of algorithms or programs libraries.

The largest mathematical formalization has been done at the University of Nijmegen (see the Constructive Coq Repository at Nijmegen).

A symbolic step has also been obtained by formalizing in full a proof of the Four Color Theorem.

#### 13 What are the industrial applications for Coq?

Coq is used e.g. to prove properties of the JavaCard system (especially by Schlumberger and Trusted Logic). It has also been used to formalize the semantics of the Lucid-Synchrone data-flow synchronous calculus used by Esterel-Technologies.

## 3 Documentation

#### 14 Where can I find documentation about Coq?

All the documentation about Coq, from the reference manual [17] to friendly tutorials [15] and documentation of the standard library, is available online. All these documents are viewable either in browsable HTML, or as downloadable postscripts.

#### 15 Where can I find this FAQ on the web?

This FAQ is available online at http://coq.inria.fr/faq.

#### 16 How can I submit suggestions / improvements / additions for this FAQ?

This FAQ is unfinished (in the sense that there are some obvious sections that are missing). Please send contributions to Coq-Club.

#### 17 Is there any mailing list about Coq?

The main Coq mailing list is coq-club@inria.fr, which broadcasts questions and suggestions about the implementation, the logical formalism or proof developments. See http://sympa.inria.fr/sympa/info/coq-club for subscription. For bugs reports see question 22.

#### 18 Where can I find an archive of the list?

The archives of the Coq mailing list are available at http://sympa.inria.fr/sympa/arc/coq-club.

#### 19 How can I be kept informed of new releases of Coq?

New versions of Coq are announced on the coq-club mailing list. If you only want to receive information about new releases, you can subscribe to Coq on http://freshmeat.net/projects/coq/.

#### 20 Is there any book about Coq?

The first book on Coq, Yves Bertot and Pierre Castéran’s Coq’Art has been published by Springer-Verlag in 2004:

> “This book provides a pragmatic introduction to the development of proofs and certified programs using Coq. With its large collection of examples and exercises it is an invaluable tool for researchers, students, and engineers interested in formal methods and the development of zero-default software.”

#### 21 Where can I find some Coq examples?

There are examples in the manual [17] and in the Coq’Art [2] exercises http://www.labri.fr/Perso/~casteran/CoqArt/index.html. You can also find large developments using Coq in the Coq user contributions: http://coq.inria.fr/contribs.

#### 22 How can I report a bug?

You can use the web interface accessible at http://coq.inria.fr, link “contacts”.

## 4 Installation

#### 23 What is the license of Coq?

Coq is distributed under the GNU Lesser General License (LGPL).

#### 24 Where can I find the sources of Coq?

The sources of Coq can be found online in the tar.gz’ed packages (http://coq.inria.fr, link “download”). Development sources can be accessed at http://coq.gforge.inria.fr/

#### 25 On which platform is Coq available?

Compiled binaries are available for Linux, MacOS X, and Windows. The sources can be easily compiled on all platforms supporting Objective Caml.

## 5 The logic of Coq

### 5.1 General

#### 26 What is the logic of Coq?

Coq is based on an axiom-free type theory called the Calculus of Inductive Constructions (see Coquand [6], Luo [16] and Coquand–Paulin-Mohring [7]). It includes higher-order functions and predicates, inductive and co-inductive datatypes and predicates, and a stratified hierarchy of sets.

#### 27 Is Coq’s logic intuitionistic or classical?

Coq’s logic is modular. The core logic is intuitionistic (i.e. excluded-middle A∨¬ A is not granted by default). It can be extended to classical logic on demand by requiring an optional module stating A∨¬ A.

#### 28 Can I define non-terminating programs in Coq?

All programs in Coq are terminating. Especially, loops must come with an evidence of their termination.

Non-terminating programs can be simulated by passing around a bound on how long the program is allowed to run before dying.

#### 29 How is equational reasoning working in Coq?

Coq comes with an internal notion of computation called conversion (e.g. (x+1)+y is internally equivalent to (x+y)+1; similarly applying argument a to a function mapping x to some expression t converts to the expression t where x is replaced by a). This notion of conversion (which is decidable because Coq programs are terminating) covers a certain part of equational reasoning but is limited to sequential evaluation of expressions of (not necessarily closed) programs. Besides conversion, equations have to be treated by hand or using specialised tactics.

### 5.2 Axioms

#### 30 What axioms can be safely added to Coq?

There are a few typical useful axioms that are independent from the Calculus of Inductive Constructions and that are considered consistent with the theory of Coq. Most of these axioms are stated in the directory Logic of the standard library of Coq. The most interesting ones are

- Excluded-middle: ∀ A:Prop, A ∨ ¬ A
- Proof-irrelevance: ∀ A:Prop ∀ p1 p2:A, p1=p2
- Unicity of equality proofs (or equivalently Streicher’s axiom K): ∀ A ∀ x y:A ∀ p1 p2:x=y, p1=p2
- Hilbert’s є operator: if A ≠ ∅, then there is єP such that ∃ x P(x) → P(єP)
- Church’s ι operator: if A ≠ ∅, then there is ιP such that ∃! x P(x) → P(ιP)
- The axiom of unique choice: ∀ x ∃! y R(x,y) → ∃ f ∀ x R(x,f(x))
- The functional axiom of choice: ∀ x ∃ y R(x,y) → ∃ f ∀ x R(x,f(x))
- Extensionality of predicates: ∀ P Q:A→ Prop, (∀ x, P(x) ↔ Q(x)) → P=Q
- Extensionality of functions: ∀ f g:A→ B, (∀ x, f(x)=g(x)) → f=g

Figure 1 is a summary of the relative strength of these axioms, most proofs can be found in directory Logic of the standard library. (Statements in boldface are the most “interesting” ones for Coq.) The justification of their validity relies on the interpretability in set theory.

> Figure 1: The dependency graph of axioms in the Calculus of Inductive Constructions

#### 31 What standard axioms are inconsistent with Coq?

The axiom of unique choice together with classical logic (e.g. excluded-middle) are inconsistent in the variant of the Calculus of Inductive Constructions where Set is impredicative.

As a consequence, the functional form of the axiom of choice and excluded-middle, or any form of the axiom of choice together with predicate extensionality are inconsistent in the Set-impredicative version of the Calculus of Inductive Constructions.

The main purpose of the Set-predicative restriction of the Calculus of Inductive Constructions is precisely to accommodate these axioms which are quite standard in mathematical usage.

The Set-predicative system is commonly considered consistent by interpreting it in a standard set-theoretic boolean model, even with classical logic, axiom of choice and predicate extensionality added.

#### 32 What is Streicher’s axiom K

Streicher’s axiom K [13] is an axiom that asserts dependent elimination of reflexive equality proofs.

Coq < Axiom Streicher_K :
forall (A:Type) (x:A) (P: x=x -> Prop),
P (eq_refl x) -> forall p: x=x, P p.

In the general case, axiom K is an independent statement of the Calculus of Inductive Constructions. However, it is true on decidable domains (see file Eqdep_dec.v). It is also trivially a consequence of proof-irrelevance (see 33) hence of classical logic.

Axiom K is equivalent to Uniqueness of Identity Proofs [13]

Coq < Axiom UIP : forall (A:Set) (x y:A) (p1 p2: x=y), p1 = p2.

Axiom K is also equivalent to Uniqueness of Reflexive Identity Proofs [13]

Coq < Axiom UIP_refl : forall (A:Set) (x:A) (p: x=x), p = eq_refl x.

Axiom K is also equivalent to

Coq < Axiom
eq_rec_eq :
forall (A:Set) (x:A) (P: A->Set) (p:P x) (h: x=x),
p = eq_rect x P p x h.

It is also equivalent to the injectivity of dependent equality (dependent equality is itself equivalent to equality of dependent pairs).

Coq < Inductive eq_dep (U:Set) (P:U -> Set) (p:U) (x:P p) :
forall q:U, P q -> Prop :=
eq_dep_intro : eq_dep U P p x p x.
Coq < Axiom
eq_dep_eq :
forall (U:Set) (u:U) (P:U -> Set) (p1 p2:P u),
eq_dep U P u p1 u p2 -> p1 = p2.

#### 33 What is proof-irrelevance

A specificity of the Calculus of Inductive Constructions is to permit statements about proofs. This leads to the question of comparing two proofs of the same proposition. Identifying all proofs of the same proposition is called proof-irrelevance:

∀ A:Prop, ∀ p q:A, p=q

Proof-irrelevance (in Prop) can be assumed without contradiction in Coq. It expresses that only provability matters, whatever the exact form of the proof is. This is in harmony with the common purely logical interpretation of Prop. Contrastingly, proof-irrelevance is inconsistent in Set since there are types in Set, such as the type of booleans, that provably have at least two distinct elements.

Proof-irrelevance (in Prop) is a consequence of classical logic (see proofs in file Classical.v and Berardi.v). Proof-irrelevance is also a consequence of propositional extensionality (i.e. (A <-> B) -> A=B, see the proof in file ClassicalFacts.v).

Proof-irrelevance directly implies Streicher’s axiom K.

#### 34 What about functional extensionality?

Extensionality of functions is admittedly consistent with the Set-predicative Calculus of Inductive Constructions.

Let A, B be types. To deal with extensionality on `A->B` without relying on a general extensionality axiom, a possible approach is to define one’s own extensional equality on `A->B`.

Coq < Definition ext_eq (f g: A->B) := forall x:A, f x = g x.

and to reason on `A->B` as a setoid (see the Chapter on Setoids in the Reference Manual).

#### 35 Is Prop impredicative?

Yes, the sort Prop of propositions is impredicative. Otherwise said, a statement of the form ∀ A:Prop, P(A) can be instantiated by itself: if ∀ A:Prop, P(A) is provable, then P(∀ A:Prop, P(A)) is.

#### 36 Is Set impredicative?

No, the sort Set lying at the bottom of the hierarchy of computational types is predicative in the basic Coq system. This means that a family of types in Set, e.g. ∀ A:Set, A → A, is not a type in Set and it cannot be applied on itself.

However, the sort Set was impredicative in the original versions of Coq. For backward compatibility, or for experiments by knowledgeable users, the logic of Coq can be set impredicative for Set by calling Coq with the option -impredicative-set.

Set has been made predicative from version 8.0 of Coq. The main reason is to interact smoothly with a classical mathematical world where both excluded-middle and the axiom of description are valid (see file ClassicalDescription.v for a proof that excluded-middle and description implies the double negation of excluded-middle in Set and file Hurkens_Set.v from the user contribution Paradoxes at http://coq.inria.fr/contribs for a proof that impredicativity of Set implies the simple negation of excluded-middle in Set).

#### 37 Is Type impredicative?

No, Type is stratified. This is hidden for the user, but Coq internally maintains a set of constraints ensuring stratification.

If Type were impredicative then it would be possible to encode Girard’s systems U− and U in Coq and it is known from Girard, Coquand, Hurkens and Miquel that systems U− and U are inconsistent [Girard 1972, Coquand 1991, Hurkens 1993, Miquel 2001]. This encoding can be found in file Logic/Hurkens.v of Coq standard library.

For instance, when the user see ∀ X:Type, X->X : Type, each occurrence of Type is implicitly bound to a different level, say α and β and the actual statement is forall X:Type(α), X->X : Type(β) with the constraint α<β.

When a statement violates a constraint, the message Universe inconsistency appears. Example: fun (x:Type) (y:∀ X:Type, X -> X) => y x x.

#### 38 I have two proofs of the same proposition. Can I prove they are equal?

In the base Coq system, the answer is generally no. However, if classical logic is set, the answer is yes for propositions in Prop. The answer is also yes if proof irrelevance holds (see question 33).

There are also “simple enough” propositions for which you can prove the equality without requiring any extra axioms. This is typically the case for propositions defined deterministically as a first-order inductive predicate on decidable sets. See for instance in question 138 an axiom-free proof of the uniqueness of the proofs of the proposition le m n (less or equal on nat).

#### 39 I have two proofs of an equality statement. Can I prove they are equal?

Yes, if equality is decidable on the domain considered (which is the case for nat, bool, etc): see Coq file `Eqdep_dec.v`). No otherwise, unless assuming Streicher’s axiom K (see [13]) or a more general assumption such as proof-irrelevance (see 33) or classical logic.

All of these statements can be found in file Eqdep.v.

#### 40 Can I prove that the second components of equal dependent pairs are equal?

The answer is the same as for proofs of equality statements. It is provable if equality on the domain of the first component is decidable (look at `inj_right_pair` from file Eqdep_dec.v), but not provable in the general case. However, it is consistent (with the Calculus of Constructions) to assume it is true. The file Eqdep.v actually provides an axiom (equivalent to Streicher’s axiom K) which entails the result (look at `inj_pair2` in Eqdep.v).

### 5.3 Impredicativity

#### 41 Why injection does not work on impredicative Set?

E.g. in this case (this occurs only in the Set-impredicative variant of Coq):

Coq < Inductive I : Type :=
intro : forall k:Set, k -> I.
Coq < Lemma eq_jdef :
forall x y:nat, intro _ x = intro _ y -> x = y.
Coq < Proof.
Coq < intros x y H; injection H.

Injectivity of constructors is restricted to predicative types. If injectivity on large inductive types were not restricted, we would be allowed to derive an inconsistency (e.g. following the lines of Burali-Forti paradox). The question remains open whether injectivity is consistent on some large inductive types not expressive enough to encode known paradoxes (such as type I above).

#### 42 What is a “large inductive definition”?

An inductive definition in Prop or Set is called large if its constructors embed sets or propositions. As an example, here is a large inductive type:

Coq < Inductive sigST (P:Set -> Set) : Type :=
existST : forall X:Set, P X -> sigST P.

In the Set impredicative variant of Coq, large inductive definitions in Set have restricted elimination schemes to prevent inconsistencies. Especially, projecting the set or the proposition content of a large inductive definition is forbidden. If it were allowed, it would be possible to encode e.g. Burali-Forti paradox [10, 5].

#### 43 Is Coq’s logic conservative over Coquand’s Calculus of Constructions?

In the Set-impredicative version of the Calculus of Inductive Constructions (CIC), there are two ways to interpret the Calculus of Constructions (CC) since the impredicative sort of CC can be interpreted either as Prop or as Set. In the Set-predicative CIC, the impredicative sort of CC can only be interpreted as Prop.

If the impredicative sort of CC is interpreted as Set, there is no conservativity of CIC over CC as the discrimination of constructors of inductive types in Set transports to a discrimination of constructors of inductive types encoded impredicatively. Concretely, considering the impredicative encoding of Boolean, equality and falsity, we can prove the following CC statement DISCR in CIC which is not provable in CC, as CC has a “term-irrelevant” model.

Coq < Definition BOOL := forall X:Set, X -> X -> X.
Coq < Definition TRUE : BOOL := fun X x1 x2 => x1.
Coq < Definition FALSE : BOOL := fun X x1 x2 => x2.
Coq < Definition EQBOOL (x1 x2:BOOL) := forall P:BOOL->Set, P x1 -> P x2.
Coq < Definition BOT := forall X:Set, X.
Coq < Definition BOOL2bool : BOOL -> bool := fun b => b bool true false.
Coq < Theorem DISCR : EQBOOL TRUE FALSE -> BOT.
Coq < intro X.
Coq < assert (H : BOOL2bool TRUE = BOOL2bool FALSE).
Coq < { apply X. trivial. }
Coq < discriminate H.
Coq < Qed.

If the impredicative sort of CC is interpreted as Prop, CIC is presumably conservative over CC. The general idea is that no proof-relevant information can flow from Prop to Set, even though singleton elimination can be used. Hence types in Set should be smashable to the unit type and Set and Type themselves be mapped to Prop.

## 6 Talkin’ with the Rooster

### 6.1 My goal is ..., how can I prove it?

#### 44 My goal is a conjunction, how can I prove it?

Use some theorem or assumption or use the split tactic.

Coq < Goal forall A B:Prop, A -> B -> A/\B.
1 subgoal
============================
forall A B : Prop, A -> B -> A /\ B
Coq < intros.
1 subgoal
A, B : Prop
H : A
H0 : B
============================
A /\ B
Coq < split.
2 subgoals
A, B : Prop
H : A
H0 : B
============================
A
subgoal 2 is:
B
Coq < assumption.
1 subgoal
A, B : Prop
H : A
H0 : B
============================
B
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 45 My goal contains a conjunction as an hypothesis, how can I use it?

If you want to decompose a hypothesis into several hypotheses, you can use the destruct tactic:

Coq < Goal forall A B:Prop, A/\B -> B.
1 subgoal
============================
forall A B : Prop, A /\ B -> B
Coq < intros.
1 subgoal
A, B : Prop
H : A /\ B
============================
B
Coq < destruct H as [H1 H2].
1 subgoal
A, B : Prop
H1 : A
H2 : B
============================
B
Coq < assumption.
No more subgoals.
Coq < Qed.

You can also perform the destruction at the time of introduction:

Coq < Goal forall A B:Prop, A/\B -> B.
1 subgoal
============================
forall A B : Prop, A /\ B -> B
Coq < intros A B [H1 H2].
1 subgoal
A, B : Prop
H1 : A
H2 : B
============================
B
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 46 My goal is a disjunction, how can I prove it?

You can prove the left part or the right part of the disjunction using left or right tactics. If you want to do a classical reasoning step, use the classic axiom to prove the right part with the assumption that the left part of the disjunction is false.

Coq < Goal forall A B:Prop, A -> A\/B.
1 subgoal
============================
forall A B : Prop, A -> A \/ B
Coq < intros.
1 subgoal
A, B : Prop
H : A
============================
A \/ B
Coq < left.
1 subgoal
A, B : Prop
H : A
============================
A
Coq < assumption.
No more subgoals.
Coq < Qed.

An example using classical reasoning:

Coq < Require Import Classical.
Coq < Ltac classical_right :=
match goal with
| _:_ |- ?X1 \/ _ => (elim (classic X1);intro;[left;trivial|right])
end.
classical_right is defined
Coq < Ltac classical_left :=
match goal with
| _:_ |- _ \/ ?X1 => (elim (classic X1);intro;[right;trivial|left])
end.
classical_left is defined
Coq < Goal forall A B:Prop, (~A -> B) -> A\/B.
1 subgoal
============================
forall A B : Prop, (~ A -> B) -> A \/ B
Coq < intros.
1 subgoal
A, B : Prop
H : ~ A -> B
============================
A \/ B
Coq < classical_right.
1 subgoal
A, B : Prop
H : ~ A -> B
H0 : ~ A
============================
B
Coq < auto.
No more subgoals.
Coq < Qed.

#### 47 My goal is an universally quantified statement, how can I prove it?

Use some theorem or assumption or introduce the quantified variable in the context using the intro tactic. If there are several variables you can use the intros tactic. A good habit is to provide names for these variables: Coq will do it anyway, but such automatic naming decreases legibility and robustness.

#### 48 My goal contains an universally quantified statement, how can I use it?

If the universally quantified assumption matches the goal you can use the apply tactic. If it is an equation you can use the rewrite tactic. Otherwise you can use the specialize tactic to instantiate the quantified variables with terms. The variant assert(Ht := H t) makes a copy of assumption H before instantiating it.

#### 49 My goal is an existential, how can I prove it?

Use some theorem or assumption or exhibit the witness using the exists tactic.

Coq < Goal exists x:nat, forall y, x+y=y.
1 subgoal
============================
exists x : nat, forall y : nat, x + y = y
Coq < exists 0.
1 subgoal
============================
forall y : nat, 0 + y = y
Coq < intros.
1 subgoal
y : nat
============================
0 + y = y
Coq < auto.
No more subgoals.
Coq < Qed.

#### 50 My goal is solvable by some lemma, how can I prove it?

Just use the apply tactic.

Coq < Lemma mylemma : forall x, x+0 = x.
1 subgoal
============================
forall x : nat, x + 0 = x
Coq < auto.
No more subgoals.
Coq < Qed.
mylemma is defined
Coq < Goal 3+0 = 3.
1 subgoal
============================
3 + 0 = 3
Coq < apply mylemma.
No more subgoals.
Coq < Qed.

#### 51 My goal contains False as an hypothesis, how can I prove it?

You can use the contradiction or intuition tactics.

#### 52 My goal is an equality of two convertible terms, how can I prove it?

Just use the reflexivity tactic.

Coq < Goal forall x, 0+x = x.
1 subgoal
============================
forall x : nat, 0 + x = x
Coq < intros.
1 subgoal
x : nat
============================
0 + x = x
Coq < reflexivity.
No more subgoals.
Coq < Qed.

#### 53 My goal is a let x := a in ..., how can I prove it?

Just use the intro tactic.

#### 54 My goal is a let (a, ..., b) := c in, how can I prove it?

Just use the destruct c as (a,...,b) tactic.

#### 55 My goal contains some existential hypotheses, how can I use it?

As with conjunctive hypotheses, you can use the destruct tactic or the intros tactic to decompose them into several hypotheses.

Coq < Require Import Arith.
Coq < Goal forall x, (exists y, x * y = 1) -> x = 1.
1 subgoal
============================
forall x : nat, (exists y : nat, x * y = 1) -> x = 1
Coq < intros x [y H].
1 subgoal
x, y : nat
H : x * y = 1
============================
x = 1
Coq < apply mult_is_one in H.
1 subgoal
x, y : nat
H : x = 1 /\ y = 1
============================
x = 1
Coq < easy.
No more subgoals.
Coq < Qed.

#### 56 My goal is an equality, how can I swap the left and right hand terms?

Just use the symmetry tactic.

Coq < Goal forall x y : nat, x=y -> y=x.
1 subgoal
============================
forall x y : nat, x = y -> y = x
Coq < intros.
1 subgoal
x, y : nat
H : x = y
============================
y = x
Coq < symmetry.
1 subgoal
x, y : nat
H : x = y
============================
x = y
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 57 My hypothesis is an equality, how can I swap the left and right hand terms?

Just use the symmetryin tactic.

Coq < Goal forall x y : nat, x=y -> y=x.
1 subgoal
============================
forall x y : nat, x = y -> y = x
Coq < intros.
1 subgoal
x, y : nat
H : x = y
============================
y = x
Coq < symmetry in H.
1 subgoal
x, y : nat
H : y = x
============================
y = x
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 58 My goal is an equality, how can I prove it by transitivity?

Just use the transitivity tactic.

Coq < Goal forall x y z : nat, x=y -> y=z -> x=z.
1 subgoal
============================
forall x y z : nat, x = y -> y = z -> x = z
Coq < intros.
1 subgoal
x, y, z : nat
H : x = y
H0 : y = z
============================
x = z
Coq < transitivity y.
2 subgoals
x, y, z : nat
H : x = y
H0 : y = z
============================
x = y
subgoal 2 is:
y = z
Coq < assumption.
1 subgoal
x, y, z : nat
H : x = y
H0 : y = z
============================
y = z
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 59 My goal would be solvable using apply;assumption if it would not create meta-variables, how can I prove it?

You can use eapply yourtheorem;eauto but it won’t work in all cases ! (for example if more than one hypothesis match one of the subgoals generated by eapply) so you should rather use try solve [eapply yourtheorem;eauto], otherwise some metavariables may be incorrectly instantiated.

Coq < Lemma trans : forall x y z : nat, x=y -> y=z -> x=z.
1 subgoal
============================
forall x y z : nat, x = y -> y = z -> x = z
Coq < intros.
1 subgoal
x, y, z : nat
H : x = y
H0 : y = z
============================
x = z
Coq < transitivity y;assumption.
No more subgoals.
Coq < Qed.
trans is defined
Coq < Goal forall x y z : nat, x=y -> y=z -> x=z.
1 subgoal
============================
forall x y z : nat, x = y -> y = z -> x = z
Coq < intros.
1 subgoal
x, y, z : nat
H : x = y
H0 : y = z
============================
x = z
Coq < eapply trans;eauto.
No more subgoals.
Coq < Qed.
Unnamed_thm5 is defined
Coq < Goal forall x y z t : nat, x=y -> x=t -> y=z -> x=z.
1 subgoal
============================
forall x y z t : nat, x = y -> x = t -> y = z -> x = z
Coq < intros.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = z
Coq < eapply trans;eauto.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
t = z
Coq < Undo.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = z
Coq < eapply trans.
2 focused subgoals
(shelved: 1)
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = ?y
subgoal 2 is:
?y = z
Coq < apply H.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
y = z
Coq < auto.
No more subgoals.
Coq < Qed.
Unnamed_thm6 is defined
Coq < Goal forall x y z t : nat, x=y -> x=t -> y=z -> x=z.
1 subgoal
============================
forall x y z t : nat, x = y -> x = t -> y = z -> x = z
Coq < intros.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = z
Coq < eapply trans;eauto.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
t = z
Coq < Undo.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = z
Coq < try solve [eapply trans;eauto].
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = z
Coq < eapply trans.
2 focused subgoals
(shelved: 1)
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
x = ?y
subgoal 2 is:
?y = z
Coq < apply H.
1 subgoal
x, y, z, t : nat
H : x = y
H0 : x = t
H1 : y = z
============================
y = z
Coq < auto.
No more subgoals.
Coq < Qed.
Unnamed_thm7 is defined

#### 60 My goal is solvable by some lemma within a set of lemmas and I don’t want to remember which one, how can I prove it?

You can use a what is called a hints’ base.

Coq < Require Import ZArith.
[Loading ML file omega_plugin.cmxs ... done]
Coq < Require Ring.
Coq < Local Open Scope Z_scope.
Coq < Lemma toto1 : 1+1 = 2.
1 subgoal
============================
1 + 1 = 2
Coq < ring.
No more subgoals.
Coq < Qed.
toto1 is defined
Coq < Lemma toto2 : 2+2 = 4.
1 subgoal
============================
2 + 2 = 4
Coq < ring.
No more subgoals.
Coq < Qed.
toto2 is defined
Coq < Lemma toto3 : 2+1 = 3.
1 subgoal
============================
2 + 1 = 3
Coq < ring.
No more subgoals.
Coq < Qed.
toto3 is defined
Coq < Hint Resolve toto1 toto2 toto3 : mybase.
Coq < Goal 2+(1+1)=4.
1 subgoal
============================
2 + (1 + 1) = 4
Coq < auto with mybase.
No more subgoals.
Coq < Qed.
Unnamed_thm8 is defined

#### 61 My goal is one of the hypotheses, how can I prove it?

Use the assumption tactic.

Coq < Goal 1=1 -> 1=1.
1 subgoal
============================
1 = 1 -> 1 = 1
Coq < intro.
1 subgoal
H : 1 = 1
============================
1 = 1
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 62 My goal appears twice in the hypotheses and I want to choose which one is used, how can I do it?

Use the exact tactic.

Coq < Goal 1=1 -> 1=1 -> 1=1.
1 subgoal
============================
1 = 1 -> 1 = 1 -> 1 = 1
Coq < intros.
1 subgoal
H, H0 : 1 = 1
============================
1 = 1
Coq < exact H0.
No more subgoals.
Coq < Qed.

#### 63 What can be the difference between applying one hypothesis or another in the context of the last question?

From a proof point of view it is equivalent but if you want to extract a program from your proof, the two hypotheses can lead to different programs.

#### 64 My goal is a propositional tautology, how can I prove it?

Just use the tauto tactic.

Coq < Goal forall A B:Prop, A-> (A\/B) /\ A.
1 subgoal
============================
forall A B : Prop, A -> (A \/ B) /\ A
Coq < intros.
1 subgoal
A, B : Prop
H : A
============================
(A \/ B) /\ A
Coq < tauto.
No more subgoals.
Coq < Qed.

#### 65 My goal is a first order formula, how can I prove it?

Just use the semi-decision tactic: firstorder.

#### 66 My goal is solvable by a sequence of rewrites, how can I prove it?

Just use the congruence tactic.

Coq < Goal forall a b c d e, a=d -> b=e -> c+b=d -> c+e=a.
1 subgoal
============================
forall a b c d e : Z, a = d -> b = e -> c + b = d -> c + e = a
Coq < intros.
1 subgoal
a, b, c, d, e : Z
H : a = d
H0 : b = e
H1 : c + b = d
============================
c + e = a
Coq < congruence.
No more subgoals.
Coq < Qed.

#### 67 My goal is a disequality solvable by a sequence of rewrites, how can I prove it?

Just use the congruence tactic.

Coq < Goal forall a b c d, a<>d -> b=a -> d=c+b -> b<>c+b.
1 subgoal
============================
forall a b c d : Z, a <> d -> b = a -> d = c + b -> b <> c + b
Coq < intros.
1 subgoal
a, b, c, d : Z
H : a <> d
H0 : b = a
H1 : d = c + b
============================
b <> c + b
Coq < congruence.
No more subgoals.
Coq < Qed.

#### 68 My goal is an equality on some ring (e.g. natural numbers), how can I prove it?

Just use the ring tactic.

Coq < Require Import ZArith.
Coq < Require Ring.
Coq < Local Open Scope Z_scope.
Coq < Goal forall a b : Z, (a+b)*(a+b) = a*a + 2*a*b + b*b.
1 subgoal
============================
forall a b : Z, (a + b) * (a + b) = a * a + 2 * a * b + b * b
Coq < intros.
1 subgoal
a, b : Z
============================
(a + b) * (a + b) = a * a + 2 * a * b + b * b
Coq < ring.
No more subgoals.
Coq < Qed.

#### 69 My goal is an equality on some field (e.g. real numbers), how can I prove it?

Just use the field tactic.

Coq < Require Import Reals.
[Loading ML file r_syntax_plugin.cmxs ... done]
[Loading ML file fourier_plugin.cmxs ... done]
[Loading ML file micromega_plugin.cmxs ... done]
Coq < Require Ring.
Coq < Local Open Scope R_scope.
Coq < Goal forall a b : R, b*a<>0 -> (a/b) * (b/a) = 1.
1 subgoal
============================
forall a b : R, b * a <> 0 -> a / b * (b / a) = 1
Coq < intros.
1 subgoal
a, b : R
H : b * a <> 0
============================
a / b * (b / a) = 1
Coq < field.
1 subgoal
a, b : R
H : b * a <> 0
============================
a <> 0 /\ b <> 0
Coq < split ; auto with real.
No more subgoals.
Coq < Qed.

#### 70 My goal is an inequality on integers in Presburger’s arithmetic (an expression build from +, −, constants, and variables), how can I prove it?

Coq < Require Import ZArith.
Coq < Require Omega.
Coq < Local Open Scope Z_scope.
Coq < Goal forall a : Z, a>0 -> a+a > a.
1 subgoal
============================
forall a : Z, a > 0 -> a + a > a
Coq < intros.
1 subgoal
a : Z
H : a > 0
============================
a + a > a
Coq < omega.
No more subgoals.
Coq < Qed.

#### 71 My goal is an equation solvable using equational hypothesis on some ring (e.g. natural numbers), how can I prove it?

You need the gb tactic (see Loïc Pottier’s homepage).

### 6.2 Tactics usage

#### 72 I want to state a fact that I will use later as an hypothesis, how can I do it?

If you want to use forward reasoning (first proving the fact and then using it) you just need to use the assert tactic. If you want to use backward reasoning (proving your goal using an assumption and then proving the assumption) use the cut tactic.

Coq < Goal forall A B C D : Prop, (A -> B) -> (B->C) -> A -> C.
1 subgoal
============================
forall A B C : Prop, Prop -> (A -> B) -> (B -> C) -> A -> C
Coq < intros.
1 subgoal
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
============================
C
Coq < assert (A->C).
2 subgoals
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
============================
A -> C
subgoal 2 is:
C
Coq < intro;apply H0;apply H;assumption.
1 subgoal
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
H2 : A -> C
============================
C
Coq < apply H2.
1 subgoal
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
H2 : A -> C
============================
A
Coq < assumption.
No more subgoals.
Coq < Qed.
Coq < Goal forall A B C D : Prop, (A -> B) -> (B->C) -> A -> C.
1 subgoal
============================
forall A B C : Prop, Prop -> (A -> B) -> (B -> C) -> A -> C
Coq < intros.
1 subgoal
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
============================
C
Coq < cut (A->C).
2 subgoals
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
============================
(A -> C) -> C
subgoal 2 is:
A -> C
Coq < intro.
2 subgoals
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
H2 : A -> C
============================
C
subgoal 2 is:
A -> C
Coq < apply H2;assumption.
1 subgoal
A, B, C, D : Prop
H : A -> B
H0 : B -> C
H1 : A
============================
A -> C
Coq < intro;apply H0;apply H;assumption.
No more subgoals.
Coq < Qed.

#### 73 I want to state a fact that I will use later as an hypothesis and prove it later, how can I do it?

You can use cut followed by intro or you can use the following Ltac command:

    Ltac assert_later t := cut t;[intro|idtac].

#### 74 What is the difference between Qed and Defined?

These two commands perform type checking, but when Defined is used the new definition is set as transparent, otherwise it is defined as opaque (see 169).

#### 75 How can I know what an automation tactic does in my example?

You can use its info variant: info_auto, info_trivial, info_eauto.

#### 76 Why auto does not work? How can I fix it?

You can increase the depth of the proof search or add some lemmas in the base of hints. Perhaps you may need to use eauto.

#### 77 What is eauto?

This is the same tactic as auto, but it relies on eapply instead of apply.

#### 78 How can I speed up auto?

You can use info_auto to replace auto by the tactics it generates. You can split your hint bases into smaller ones.

#### 79 What is the equivalent of tauto for classical logic?

Currently there are no equivalent tactic for classical logic. You can use Gödel’s “not not” translation.

#### 80 I want to replace some term with another in the goal, how can I do it?

If one of your hypothesis (say H) states that the terms are equal you can use the rewrite tactic. Otherwise you can use the replace with tactic.

#### 81 I want to replace some term with another in an hypothesis, how can I do it?

You can use the rewrite in tactic.

#### 82 I want to replace some symbol with its definition, how can I do it?

You can use the unfold tactic.

#### 83 How can I reduce some term?

You can use the simpl tactic.

#### 84 How can I declare a shortcut for some term?

You can use the set or pose tactics.

#### 85 How can I perform case analysis?

You can use the case or destruct tactics.

#### 86 How can I prevent the case tactic from losing information ?

You may want to use the (now standard) case_eq tactic. See the Coq’Art page 159.

#### 87 Why should I name my intros?

When you use the intro tactic you don’t have to give a name to your hypothesis. If you do so the name will be generated by Coq but your scripts may be less robust. If you add some hypothesis to your theorem (or change their order), you will have to change your proof to adapt to the new names.

#### 88 How can I automatize the naming?

You can use the Show Intro. or Show Intros. commands to generate the names and use your editor to generate a fully named intro tactic. This can be automatized within xemacs.

Coq < Goal forall A B C : Prop, A -> B -> C -> A/\B/\C.
1 subgoal
============================
forall A B C : Prop, A -> B -> C -> A /\ B /\ C
Coq < Show Intros.
A B C H H0 H1
Coq < (*
A B C H H0
H1
*)
intros A B C H H0 H1.
1 subgoal
A, B, C : Prop
H : A
H0 : B
H1 : C
============================
A /\ B /\ C
Coq < repeat split;assumption.
No more subgoals.
Coq < Qed.

#### 89 I want to automatize the use of some tactic, how can I do it?

You need to use the proof with T command and add … at the end of your sentences.

For instance:

Coq < Goal forall A B C : Prop, A -> B/\C -> A/\B/\C.
1 subgoal
============================
forall A B C : Prop, A -> B /\ C -> A /\ B /\ C
Coq < Proof with assumption.
1 subgoal
============================
forall A B C : Prop, A -> B /\ C -> A /\ B /\ C
Coq < intros.
1 subgoal
A, B, C : Prop
H : A
H0 : B /\ C
============================
A /\ B /\ C
Coq < split...
No more subgoals.
Coq < Qed.

#### 90 I want to execute the proof with tactic only if it solves the goal, how can I do it?

You need to use the try and solve tactics. For instance:

Coq < Require Import ZArith.
Coq < Require Ring.
Coq < Local Open Scope Z_scope.
Coq < Goal forall a b c : Z, a+b=b+a.
1 subgoal
============================
forall a b : Z, Z -> a + b = b + a
Coq < Proof with try solve [ring].
1 subgoal
============================
forall a b : Z, Z -> a + b = b + a
Coq < intros...
No more subgoals.
Coq < Qed.

#### 91 How can I do the opposite of the intro tactic?

You can use the generalize tactic.

Coq < Goal forall A B : Prop, A->B-> A/\B.
1 subgoal
============================
forall A B : Prop, A -> B -> A /\ B
Coq < intros.
1 subgoal
A, B : Prop
H : A
H0 : B
============================
A /\ B
Coq < generalize H.
1 subgoal
A, B : Prop
H : A
H0 : B
============================
A -> A /\ B
Coq < intro.
1 subgoal
A, B : Prop
H : A
H0 : B
H1 : A
============================
A /\ B
Coq < auto.
No more subgoals.
Coq < Qed.

#### 92 One of the hypothesis is an equality between a variable and some term, I want to get rid of this variable, how can I do it?

You can use the subst tactic. This will rewrite the equality everywhere and clear the assumption.

#### 93 What can I do if I get “generated subgoal term has metavariables in it ”?

You should use the eapply tactic, this will generate some goals containing metavariables.

#### 94 How can I instantiate some metavariable?

Just use the instantiate tactic.

#### 95 What is the use of the pattern tactic?

The pattern tactic transforms the current goal, performing beta-expansion on all the applications featuring this tactic’s argument. For instance, if the current goal includes a subterm phi(t), then pattern t transforms the subterm into (fun x:A => phi(x)) t. This can be useful when apply fails on matching, to abstract the appropriate terms.

#### 96 What is the difference between assert, cut and generalize?

PS: Notice for people that are interested in proof rendering that assertand pose (and cut) are not rendered the same as generalize (see the HELM experimental rendering tool at http://helm.cs.unibo.it, link HELM, link COQ Online). Indeed generalize builds a beta-expanded term while assert, pose and cut uses a let-in.

      (* Goal is T *)
      generalize (H1 H2).
      (* Goal is A->T *)
      ... a proof of A->T ...

is rendered into something like

      (h) ... the proof of A->T ...
          we proved A->T
      (h0) by (H1 H2) we proved A
      by (h h0) we proved T

while

      (* Goal is T *)
      assert q := (H1 H2).
      (* Goal is A *)
      ... a proof of A ...
      (* Goal is A |- T *)
      ... a proof of T ...

is rendered into something like

      (q) ... the proof of A ...
          we proved A
      ... the proof of T ...
      we proved T

Otherwise said, generalize is not rendered in a forward-reasoning way, while assert is.

#### 97 What can I do if Coqcan not infer some implicit argument ?

You can state explicitly what this implicit argument is. See 98.

#### 98 How can I explicit some implicit argument ?

Just use A:=term where A is the argument.

For instance if you want to use the existence of “nil” on nat*nat lists:

    exists (nil (A:=(nat*nat))).

### 6.3 Proof management

#### 99 How can I change the order of the subgoals?

You can use the Focus command to concentrate on some goal. When the goal is proved you will see the remaining goals.

#### 100 How can I change the order of the hypothesis?

You can use the Move ... after command.

#### 101 How can I change the name of an hypothesis?

You can use the Rename ... into command.

#### 102 How can I delete some hypothesis?

You can use the Clear command.

#### 103 How can use a proof which is not finished?

You can use the Admitted command to state your current proof as an axiom. You can use the give_up tactic to omit a portion of a proof.

#### 104 How can I state a conjecture?

You can use the Admitted command to state your current proof as an axiom.

#### 105 What is the difference between a lemma, a fact and a theorem?

From Coq point of view there are no difference. But some tools can have a different behavior when you use a lemma rather than a theorem. For instance coqdoc will not generate documentation for the lemmas within your development.

#### 106 How can I organize my proofs?

You can organize your proofs using the section mechanism of Coq. Have a look at the manual for further information.

## 7 Inductive and Co-inductive types

### 7.1 General

#### 107 How can I prove that two constructors are different?

You can use the discriminate tactic.

Coq < Inductive toto : Set := | C1 : toto | C2 : toto.
toto is defined
toto_rect is defined
toto_ind is defined
toto_rec is defined
Coq < Goal C1 <> C2.
1 subgoal
============================
C1 <> C2
Coq < discriminate.
No more subgoals.
Coq < Qed.

#### 108 During an inductive proof, how to get rid of impossible cases of an inductive definition?

Use the inversion tactic.

#### 109 How can I prove that 2 terms in an inductive set are equal? Or different?

Have a look at decide equality and discriminate in the Reference Manual.

#### 110 Why is the proof of 0+n=n on natural numbers trivial but the proof of n+0=n is not?

Since + (plus) on natural numbers is defined by analysis on its first argument

Coq < Print plus.
Notation plus := Init.Nat.add

The expression 0+n evaluates to n. As Coq reasons modulo evaluation of expressions, 0+n and n are considered equal and the theorem 0+n=n is an instance of the reflexivity of equality. On the other side, n+0 does not evaluate to n and a proof by induction on n is necessary to trigger the evaluation of +.

#### 111 Why is dependent elimination in Prop not available by default?

This is just because most of the time it is not needed. To derive a dependent elimination principle in Prop, use the command Scheme and apply the elimination scheme using the `using` option of `elim`, `destruct` or `induction`.

#### 112 Argh! I cannot write expressions like “ if n <= p then p else n ”, as in any programming language

The short answer : You should use le_lt_dec n p instead.
The long answer: That’s right, you can’t. If you type for instance the following “definition”:

Coq < Fail Definition max (n p : nat) := if n <= p then p else n.
The command has indeed failed with message:
The term "n <= p" has type "Prop" which is not a (co-)inductive type.

As Coq says, the term “ n <= p ” is a proposition, i.e. a statement that belongs to the mathematical world. There are many ways to prove such a proposition, either by some computation, or using some already proven theorems. For instance, proving 3−2 ≤ 245503 is very easy, using some theorems on arithmetical operations. If you compute both numbers before comparing them, you risk to use a lot of time and space.

On the contrary, a function for computing the greatest of two natural numbers is an algorithm which, called on two natural numbers n and p, determines whether n≤ p or p < n. Such a function is a decision procedure for the inequality of nat. The possibility of writing such a procedure comes directly from de decidability of the order ≤ on natural numbers.

When you write a piece of code like “ if n <= p then … else … ” in a programming language like ML or Java, a call to such a decision procedure is generated. The decision procedure is in general a primitive function, written in a low-level language, in the correctness of which you have to trust.

The standard Library of the system Coq contains a (constructive) proof of decidability of the order ≤ on nat : the function le_lt_dec of the module Compare_dec of library Arith.

The following code shows how to define correctly min and max, and prove some properties of these functions.

Coq < Require Import Compare_dec.
Coq < Definition max (n p : nat) := if le_lt_dec n p then p else n.
max is defined
Coq < Definition min (n p : nat) := if le_lt_dec n p then n else p.
min is defined
Coq < Eval compute in (min 4 7).
= 4
: nat
Coq < Theorem min_plus_max : forall n p, min n p + max n p = n + p.
1 subgoal
============================
forall n p : nat, min n p + max n p = n + p
Coq < Proof.
1 subgoal
============================
forall n p : nat, min n p + max n p = n + p
Coq < intros n p;
unfold min, max;
case (le_lt_dec n p);
simpl; auto with arith.
No more subgoals.
Coq < Qed.
min_plus_max is defined
Coq < Theorem max_equiv : forall n p, max n p = p <-> n <= p.
1 subgoal
============================
forall n p : nat, max n p = p <-> n <= p
Coq < Proof.
1 subgoal
============================
forall n p : nat, max n p = p <-> n <= p
Coq < unfold max; intros n p; case (le_lt_dec n p);simpl; auto.
2 subgoals
n, p : nat
============================
n <= p -> p = p <-> n <= p
subgoal 2 is:
p < n -> n = p <-> n <= p
Coq < intuition auto with arith.
1 subgoal
n, p : nat
============================
p < n -> n = p <-> n <= p
Coq < split.
2 subgoals
n, p : nat
l : p < n
============================
n = p -> n <= p
subgoal 2 is:
n <= p -> n = p
Coq < intro e; rewrite e; auto with arith.
1 subgoal
n, p : nat
l : p < n
============================
n <= p -> n = p
Coq < intro H; absurd (p < p); eauto with arith.
No more subgoals.
Coq < Qed.
max_equiv is defined

#### 113 I wrote my own decision procedure for ≤, which is much faster than yours, but proving such theorems as max_equiv seems to be quite difficult

Your code is probably the following one:

Coq < Fixpoint my_le_lt_dec (n p :nat) {struct n}: bool :=
match n, p with 0, _ => true
| S n', S p' => my_le_lt_dec n' p'
| _ , _ => false
end.
my_le_lt_dec is defined
my_le_lt_dec is recursively defined (decreasing on 1st argument)
Coq < Definition my_max (n p:nat) := if my_le_lt_dec n p then p else n.
my_max is defined
Coq < Definition my_min (n p:nat) := if my_le_lt_dec n p then n else p.
my_min is defined

For instance, the computation of my_max 567 321 is almost immediate, whereas one can’t wait for the result of max 56 32, using Coq’s le_lt_dec.

This is normal. Your definition is a simple recursive function which returns a boolean value. Coq’s le_lt_dec is a certified function, i.e. a complex object, able not only to tell whether n≤ p or p<n, but also of building a complete proof of the correct inequality. What make le_lt_dec inefficient for computing min and max is the building of a huge proof term.

Nevertheless, le_lt_dec is very useful. Its type is a strong specification, using the sumbool type (look at the reference manual or chapter 9 of [1]). Eliminations of the form “ case (le_lt_dec n p) ” provide proofs of either n ≤ p or p < n, allowing easy proofs of some theorems as in question 112. Unfortunately, this not the case of your my_le_lt_dec, which returns a quite non-informative boolean value.

Coq < Check le_lt_dec.
le_lt_dec
: forall n m : nat, {n <= m} + {m < n}

You should keep in mind that le_lt_dec is useful to build certified programs which need to compare natural numbers, and is not designed to compare quickly two numbers.

Nevertheless, the extraction of le_lt_dec towards OCaml or Haskell, is a reasonable program for comparing two natural numbers in Peano form in linear time.

It is also possible to keep your boolean function as a decision procedure, but you have to establish yourself the relationship between my_le_lt_dec and the propositions n≤ p and p<n:

Coq < Theorem my_le_lt_dec_true :
forall n p, my_le_lt_dec n p = true <-> n <= p.
Coq < Theorem my_le_lt_dec_false :
forall n p, my_le_lt_dec n p = false <-> p < n.

### 7.2 Recursion

#### 114 Why can’t I define a non terminating program?

Because otherwise the decidability of the type-checking algorithm (which involves evaluation of programs) is not ensured. On another side, if non terminating proofs were allowed, we could get a proof of False:

Coq < (* This is fortunately not allowed! *)
Fixpoint InfiniteProof (n:nat) : False := InfiniteProof n.
Coq < Theorem Paradox : False.
Coq < Proof (InfiniteProof O).

#### 115 Why only structurally well-founded loops are allowed?

The structural order on inductive types is a simple and powerful notion of termination. The consistency of the Calculus of Inductive Constructions relies on it and another consistency proof would have to be made for stronger termination arguments (such as the termination of the evaluation of CIC programs themselves!).

In spite of this, all non-pathological termination orders can be mapped to a structural order. Tools to do this are provided in the file Wf.v of the standard library of Coq.

#### 116 How to define loops based on non structurally smaller recursive calls?

The procedure is as follows (we consider the definition of mergesort as an example).

- Define the termination order, say R on the type A of the arguments of the loop.
  Coq < Definition R (a b:list nat) := length a < length b.
- Prove that this order is well-founded (in fact that all elements in A are accessible along R).
  Coq < Lemma Rwf : well_founded R.
- Define the step function (which needs proofs that recursive calls are on smaller arguments).
  Coq < Definition split (l : list nat)
  : {l1: list nat | R l1 l} * {l2 : list nat | R l2 l}.
  Coq < Admitted.
  Coq < Definition concat (l1 l2 : list nat) : list nat.
  Coq < Admitted.
  Coq < Definition merge_step (l : list nat) (f: forall l':list nat, R l' l -> list nat) :=
  let (lH1,lH2) := (split l) in
  let (l1,H1) := lH1 in
  let (l2,H2) := lH2 in
  concat (f l1 H1) (f l2 H2).
- Define the recursive function by fixpoint on the step function.
  Coq < Definition merge := Fix Rwf (fun _ => list nat) merge_step.

#### 117 What is behind the accessibility and well-foundedness proofs?

Well-foundedness of some relation R on some type A is defined as the accessibility of all elements of A along R.

Coq < Print well_founded.
well_founded =
fun (A : Type) (R : A -> A -> Prop) => forall a : A, Acc R a
: forall A : Type, (A -> A -> Prop) -> Prop
Argument A is implicit
Argument scopes are [type_scope function_scope]
Coq < Print Acc.
Inductive Acc (A : Type) (R : A -> A -> Prop) (x : A) : Prop :=
Acc_intro : (forall y : A, R y x -> Acc R y) -> Acc R x
For Acc: Argument A is implicit
For Acc_intro: Arguments A, R are implicit
For Acc: Argument scopes are [type_scope function_scope _]
For Acc_intro: Argument scopes are [type_scope function_scope _
function_scope]

The structure of the accessibility predicate is a well-founded tree branching at each node x in A along all the nodes x’ less than x along R. Any sequence of elements of A decreasing along the order R are branches in the accessibility tree. Hence any decreasing along R is mapped into a structural decreasing in the accessibility tree of R. This is emphasised in the definition of fix which recurs not on its argument x:A but on the accessibility of this argument along R.

See file Wf.v.

#### 118 How to perform simultaneous double induction?

In general a (simultaneous) double induction is simply solved by an induction on the first hypothesis followed by an inversion over the second hypothesis. Here is an example

Coq < Inductive even : nat -> Prop :=
| even_O : even 0
| even_S : forall n:nat, even n -> even (S (S n)).
even is defined
even_ind is defined
Coq < Inductive odd : nat -> Prop :=
| odd_SO : odd 1
| odd_S : forall n:nat, odd n -> odd (S (S n)).
odd is defined
odd_ind is defined
Coq < Lemma not_even_and_odd : forall n:nat, even n -> odd n -> False.
1 subgoal
============================
forall n : nat, even n -> odd n -> False
Coq < induction 1.
2 subgoals
============================
odd 0 -> False
subgoal 2 is:
odd (S (S n)) -> False
Coq < inversion 1.
1 subgoal
n : nat
H : even n
IHeven : odd n -> False
============================
odd (S (S n)) -> False
Coq < inversion 1. apply IHeven; trivial.
1 subgoal
n : nat
H : even n
IHeven : odd n -> False
H0 : odd (S (S n))
n0 : nat
H2 : odd n
H1 : n0 = n
============================
False
No more subgoals.
Coq < Qed.

In case the type of the second induction hypothesis is not dependent, inversion can just be replaced by destruct.

#### 119 How to define a function by simultaneous double recursion?

The same trick applies, you can even use the pattern-matching compilation algorithm to do the work for you. Here is an example:

Coq < Fixpoint minus (n m:nat) {struct n} : nat :=
match n, m with
| O, _ => 0
| S k, O => S k
| S k, S l => minus k l
end.
minus is defined
minus is recursively defined (decreasing on 1st argument)
Coq < Print minus.
minus =
fix minus (n m : nat) {struct n} : nat :=
match n with
| 0 => 0
| S k => match m with
| 0 => S k
| S l => minus k l
end
end
: nat -> nat -> nat
Argument scopes are [nat_scope nat_scope]

In case of dependencies in the type of the induction objects t1 and t2, an extra argument stating t1=t2 must be given to the fixpoint definition

#### 120 How to perform nested and double induction?

To reason by nested (i.e. lexicographic) induction, just reason by induction on the successive components.

Double induction (or induction on pairs) is a restriction of the lexicographic induction. Here is an example of double induction.

Coq < Lemma nat_double_ind :
forall P : nat -> nat -> Prop, P 0 0 ->
(forall m n, P m n -> P m (S n)) ->
(forall m n, P m n -> P (S m) n) ->
forall m n, P m n.
1 subgoal
============================
forall P : nat -> nat -> Prop,
P 0 0 ->
(forall m n : nat, P m n -> P m (S n)) ->
(forall m n : nat, P m n -> P (S m) n) -> forall m n : nat, P m n
Coq < intros P H00 HmS HSn; induction m.
2 subgoals
P : nat -> nat -> Prop
H00 : P 0 0
HmS : forall m n : nat, P m n -> P m (S n)
HSn : forall m n : nat, P m n -> P (S m) n
============================
forall n : nat, P 0 n
subgoal 2 is:
forall n : nat, P (S m) n
Coq < (* case 0 *)
induction n; [assumption | apply HmS; apply IHn].
1 subgoal
P : nat -> nat -> Prop
H00 : P 0 0
HmS : forall m n : nat, P m n -> P m (S n)
HSn : forall m n : nat, P m n -> P (S m) n
m : nat
IHm : forall n : nat, P m n
============================
forall n : nat, P (S m) n
Coq < (* case Sm *)
intro n; apply HSn; apply IHm.
No more subgoals.
Coq < Qed.

#### 121 How to define a function by nested recursion?

The same trick applies. Here is the example of Ackermann function.

Coq < Fixpoint ack (n:nat) : nat -> nat :=
match n with
| O => S
| S n' =>
(fix ack' (m:nat) : nat :=
match m with
| O => ack n' 1
| S m' => ack n' (ack' m')
end)
end.
ack is defined
ack is recursively defined (decreasing on 1st argument)

### 7.3 Co-inductive types

#### 122 I have a cofixpoint t:=F(t) and I want to prove t=F(t). How to do it?

Just case-expand F(t) then complete by a trivial case analysis. Here is what it gives on e.g. the type of streams on naturals

Coq < CoInductive Stream (A:Set) : Set :=
Cons : A -> Stream A -> Stream A.
Stream is defined
Coq < CoFixpoint nats (n:nat) : Stream nat := Cons n (nats (S n)).
nats is defined
nats is corecursively defined
Coq < Lemma Stream_unfold :
forall n:nat, nats n = Cons n (nats (S n)).
1 subgoal
============================
forall n : nat, nats n = Cons n (nats (S n))
Coq < Proof.
1 subgoal
============================
forall n : nat, nats n = Cons n (nats (S n))
Coq < intro;
change (nats n = match nats n with
| Cons x s => Cons x s
end).
1 subgoal
n : nat
============================
nats n = match nats n with
| Cons x s => Cons x s
end
Coq < case (nats n); reflexivity.
No more subgoals.
Coq < Qed.

## 8 Syntax and notations

#### 123 I do not want to type “forall” because it is too long, what can I do?

You can define your own notation for forall:

    Notation "fa x : t, P" := (forall x:t, P) (at level 200, x ident).

or if your are using CoqIde you can define a pretty symbol for for all and an input method (see 152).

#### 124 How can I define a notation for square?

You can use for instance:

    Notation "x ^2" := (Rmult x x) (at level 20).

Note that you can not use: Notation "x 2" := (Rmult x x) (at level 20). because “2” is an iso-latin character. If you really want this kind of notation you should use UTF-8.

#### 125 Why “no associativity” and “left associativity” at the same level does not work?

Because we relie on Camlp4 for syntactical analysis and Camlp4 does not really implement no associativity. By default, non associative operators are defined as right associative.

#### 126 How can I know the associativity associated with a level?

You can do “Print Grammar constr”, and decode the output from Camlp4, good luck !

## 9 Modules

## 10 Ltac

#### 127 What is Ltac?

Ltac is the tactic language for Coq. It provides the user with a high-level “toolbox” for tactic creation.

#### 128 Is there any printing command in Ltac?

You can use the idtac tactic with a string argument. This string will be printed out. The same applies to the fail tactic

#### 129 What is the syntax for let in Ltac?

If xi are identifiers and ei and expr are tactic expressions, then let reads:

let x1:=e1 with x2:=e2…with xn:=en in expr.

Beware that if expr is complex (i.e. features at least a sequence) parenthesis should be added around it. For example:

Coq < Ltac twoIntro := let x:=intro in (x;x).
twoIntro is defined

#### 130 What is the syntax for pattern matching in Ltac?

Pattern matching on a term expr (non-linear first order unification) with patterns pi and tactic expressions ei reads:

match expr with p1 => e1 |p2 => e2 … |pn => en | _ => en+1 end.

Underscore matches all terms.

#### 131 What is the semantics for “match goal”?

The semantics of match goal depends on whether it returns tactics or not. The match goal expression matches the current goal against a series of patterns: hyp1 … hypn |- ccl. It uses a first-order unification algorithm and in case of success, if the right-hand-side is an expression, it tries to type it while if the right-hand-side is a tactic, it tries to apply it. If the typing or the tactic application fails, the match goal tries all the possible combinations of hypi before dropping the branch and moving to the next one. Underscore matches all terms.

#### 132 Why can’t I use a “match goal” returning a tactic in a non tail-recursive position?

This is precisely because the semantics of match goal is to apply the tactic on the right as soon as a pattern unifies what is meaningful only in tail-recursive uses.

The semantics in non tail-recursive call could have been the one used for terms (i.e. fail if the tactic expression is not typable, but don’t try to apply it). For uniformity of semantics though, this has been rejected.

#### 133 How can I generate a new name?

You can use the following syntax: let id:=fresh in …
For example:

Coq < Ltac introIdGen := let id:=fresh in intro id.
introIdGen is defined

## 11 Tactics written in OCaml

#### 134 Can you show me an example of a tactic written in OCaml?

Have a look at the skeleton “Hello World” tactic from the next question. You also have some examples of tactics written in OCaml in the “plugins” directory of Coq sources.

#### 135 Is there a skeleton of OCaml tactic I can reuse somewhere?

The following steps describe how to write a simplistic “Hello world” OCaml tactic. This takes the form of a dynamically loadable OCaml module, which will be invoked from the Coq toplevel.

1. In the `plugins` directory of the Coq source location, create a directory `hello`. Proceed to create a grammar and OCaml file, respectively `plugins/hello/g_hello.ml4` and `plugins/hello/coq_hello.ml`, containing:
   - in `g_hello.ml4`:

         (*i camlp4deps: "grammar/grammar.cma" i*)
         TACTIC EXTEND Hello
         | [ "hello" ] -> [ Coq_hello.printHello ]
         END

   - in `coq_hello.ml`:

         let printHello gl =
         Tacticals.tclIDTAC_MESSAGE (Pp.str "Hello world") gl

2. Create a file `plugins/hello/hello_plugin.mllib`, containing the names of the OCaml modules bundled in the dynamic library:

       Coq_hello
       G_hello

3. Append the following lines in `plugins/plugins{byte,opt}.itarget`:
   - in `pluginsopt.itarget`:

         hello/hello_plugin.cmxa

   - in `pluginsbyte.itarget`:

         hello/hello_plugin.cma

4. In the root directory of the Coq source location, modify the file `Makefile.common`:
   - add `hello` to the `SRCDIR` definition (second argument of the `addprefix` function);
   - in the section “Object and Source files”, add `HELLOCMA:=plugins/hello/hello_plugin.cma`;
   - add `$(HELLOCMA)` to the `PLUGINSCMA` definition.
5. Modify the file `Makefile.build`, adding in section “3) plugins” the line:

       hello: $(HELLOCMA)

6. From the command line, run `make hello`, then `make plugins/hello/hello_plugin.cmxs`.

The call to the tactic `hello` from a Coq script has to be preceded by `Declare ML Module "hello_plugin"`, which will load the dynamic object `hello_plugin.cmxs`. For instance:

    Declare ML Module "hello_plugin".
    Variable A:Prop.
    Goal A-> A.
    Proof.
    hello.
    auto.
    Qed.

## 12 Case studies

#### 136 How to prove that 2 sets are different?

You need to find a property true on one set and false on the other one. As an example we show how to prove that bool and nat are discriminable. As discrimination property we take the property to have no more than 2 elements.

Coq < Theorem nat_bool_discr : bool <> nat.
Coq < Proof.
Coq < pose (discr :=
fun X:Set =>
~ (forall a b:X, ~ (forall x:X, x <> a -> x <> b -> False))).
Coq < intro Heq; assert (H: discr bool).
Coq < intro H; apply (H true false); destruct x; auto.
Coq < rewrite Heq in H; apply H; clear H.
Coq < destruct a; destruct b as [|n]; intro H0; eauto.
Coq < destruct n; [ apply (H0 2); discriminate | eauto ].
Coq < Qed.

#### 137 Is there an axiom-free proof of Streicher’s axiom K for the equality on nat?

Yes, because equality is decidable on nat. Here is the proof.

Coq < Require Import Eqdep_dec.
Coq < Require Import Peano_dec.
Coq < Theorem K_nat :
forall (x:nat) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Coq < Proof.
Coq < intros; apply K_dec_set with (p := p).
Coq < apply eq_nat_dec.
Coq < assumption.
Coq < Qed.

Similarly, we have

Coq < Theorem eq_rect_eq_nat :
forall (p:nat) (Q:nat->Type) (x:Q p) (h:p=p), x = eq_rect p Q x p h.
Coq < Proof.
Coq < intros; apply K_nat with (p := h); reflexivity.
Coq < Qed.

#### 138 How to prove that two proofs of n<=m on nat are equal?

This is provable without requiring any axiom because axiom K directly holds on nat. Here is a proof using question 137.

Coq < Require Import Arith.
Coq < Scheme le_ind' := Induction for le Sort Prop.
Coq < Theorem le_uniqueness_proof : forall (n m : nat) (p q : n <= m), p = q.
Coq < Proof.
Coq < induction p using le_ind'; intro q.
Coq < replace (le_n n) with
(eq_rect _ (fun n0 => n <= n0) (le_n n) _ eq_refl).
Coq < 2:reflexivity.
Coq < generalize (eq_refl n).
Coq < pattern n at 2 4 6 10, q; case q; [intro | intros m l e].
Coq < rewrite <- eq_rect_eq_nat; trivial.
Coq < contradiction (le_Sn_n m); rewrite <- e; assumption.
Coq < replace (le_S n m p) with
(eq_rect _ (fun n0 => n <= n0) (le_S n m p) _ eq_refl).
Coq < 2:reflexivity.
Coq < generalize (eq_refl (S m)).
Coq < pattern (S m) at 1 3 4 6, q; case q; [intro Heq | intros m0 l HeqS].
Coq < contradiction (le_Sn_n m); rewrite Heq; assumption.
Coq < injection HeqS; intro Heq; generalize l HeqS.
Coq < rewrite <- Heq; intros; rewrite <- eq_rect_eq_nat.
Coq < rewrite (IHp l0); reflexivity.
Coq < Qed.

#### 139 How to exploit equalities on sets

To extract information from an equality on sets, you need to find a predicate of sets satisfied by the elements of the sets. As an example, let’s consider the following theorem.

Coq < Theorem interval_discr :
forall m n:nat,
{x : nat | x <= m} = {x : nat | x <= n} -> m = n.

We have a proof requiring the axiom of proof-irrelevance. We conjecture that proof-irrelevance can be circumvented by introducing a primitive definition of discrimination of the proofs of `{x : nat | x <= m}`.

Here is the proof.

#### 140 I have a problem of dependent elimination on proofs, how to solve it?

Coq < Inductive Def1 : Set := c1 : Def1.
Coq < Inductive DefProp : Def1 -> Prop :=
c2 : forall d:Def1, DefProp d.
Coq < Inductive Comb : Set :=
c3 : forall d:Def1, DefProp d -> Comb.
Coq < Lemma eq_comb :
forall (d1 d1':Def1) (d2:DefProp d1) (d2':DefProp d1'),
d1 = d1' -> c3 d1 d2 = c3 d1' d2'.

You need to derive the dependent elimination scheme for DefProp by hand using Scheme.

Coq < Scheme DefProp_elim := Induction for DefProp Sort Prop.
Coq < Lemma eq_comb :
forall d1 d1':Def1,
d1 = d1' ->
forall (d2:DefProp d1) (d2':DefProp d1'), c3 d1 d2 = c3 d1' d2'.
Coq < intros.
Coq < destruct H.
Coq < destruct d2 using DefProp_elim.
Coq < destruct d2' using DefProp_elim.
Coq < reflexivity.
Coq < Qed.

#### 141 And what if I want to prove the following?

Coq < Inductive natProp : nat -> Prop :=
| p0 : natProp 0
| pS : forall n:nat, natProp n -> natProp (S n).
Coq < Inductive package : Set :=
pack : forall n:nat, natProp n -> package.
Coq < Lemma eq_pack :
forall n n':nat,
n = n' ->
forall (np:natProp n) (np':natProp n'), pack n np = pack n' np'.
Coq < Scheme natProp_elim := Induction for natProp Sort Prop.
Coq < Definition pack_S : package -> package.
Coq < destruct 1.
Coq < apply (pack (S n)).
Coq < apply pS; assumption.
Coq < Defined.
Coq < Lemma eq_pack :
forall n n':nat,
n = n' ->
forall (np:natProp n) (np':natProp n'), pack n np = pack n' np'.
Coq < intros n n' Heq np np'.
Coq < generalize dependent n'.
Coq < induction np using natProp_elim.
Coq < induction np' using natProp_elim; intros; auto.
Coq < discriminate Heq.
Coq < induction np' using natProp_elim; intros; auto.
Coq < discriminate Heq.
Coq < change (pack_S (pack n np) = pack_S (pack n0 np')).
Coq < apply (f_equal (A:=package)).
Coq < apply IHnp.
Coq < auto.
Coq < Qed.

## 13 Publishing tools

#### 142 How can I generate some latex from my development?

You can use coqdoc.

#### 143 How can I generate some HTML from my development?

You can use coqdoc.

#### 144 How can I generate some dependency graph from my development?

You can use the tool `coqgraph` developed by Philippe Audebaud in 2002. This tool transforms dependencies generated by `coqdep` into ’dot’ files which can be visualized using the Graphviz software (http://www.graphviz.org/).

#### 145 How can I cite some Coq in my latex document?

You can use coq_tex.

#### 146 How can I cite the Coq reference manual?

You can use this bibtex entry (to adapt to the appropriate version):

    @manual{Coq:manual,
      author      = {{Coq} {Development} {Team}, The},
      title       = {The {Coq} Proof Assistant Reference Manual, version 8.7},
      month       = Oct,
      year        = {2017},
      url         = {http://coq.inria.fr}
    }

#### 147 Where can I publish my developments in Coq?

You can submit your developments as a user contribution to the Coq development team. This ensures its liveness along the evolution and possible changes of Coq.

You can also submit your developments to the HELM/MoWGLI repository at the University of Bologna (see http://mowgli.cs.unibo.it). For developments submitted in this database, it is possible to visualize the developments in natural language and execute various retrieving requests.

#### 148 How can I read my proof in natural language?

You can submit your proof to the HELM/MoWGLI repository and use the rendering tool provided by the server (see http://mowgli.cs.unibo.it).

## 14 CoqIde

#### 149 What is CoqIde?

CoqIde is a gtk based GUI for Coq.

#### 150 How to enable Emacs keybindings?

If in Gnome, run the gnome configuration editor (gconf-editor) and set key gtk-key-theme to Emacs in the category desktop/gnome/interface.

Otherwise, you need to find where the `gtk-key-theme-name` option is located in your configuration, and set it to Emacs. Usually, it is in the `$(HOME)/.gtkrc-2.0` file.

#### 151 How to enable antialiased fonts?

Set the `GDK_USE_XFT` variable to `1`. This is by default with `Gtk >= 2.2`. If some of your fonts are not available, set `GDK_USE_XFT` to `0`.

#### 152 How to use those Forall and Exists pretty symbols?

Thanks to the notation features in Coq, you just need to insert these lines in your Coq buffer:
Notation "∀ x : t, P" := (forall x:t, P) (at level 200, x ident).
Notation "∃ x : t, P" := (exists x:t, P) (at level 200, x ident).

Copy/Paste of these lines from this file will not work outside of CoqIde. You need to load a file containing these lines or to enter the ∀ using an input method (see 153). To try it just use `Require Import utf8` from inside CoqIde. To enable these notations automatically start coqide with

     coqide -l utf8

In the ide subdir of Coq library, you will find a sample utf8.v with some pretty simple notations.

#### 153 How to define an input method for non ASCII symbols?

- First solution: type `<CONTROL><SHIFT>2200` to enter a forall in the script widow. 2200 is the hexadecimal code for forall in unicode charts and is encoded as in UTF-8. 2203 is for exists. See http://www.unicode.org for more codes.
- Second solution: rebind `<AltGr>a` to forall and `<AltGr>e` to exists.

  Under X11, one can add those lines in the file /.xmodmaprc :

      ! forall
      keycode  24 = a A a A U2200 NoSymbol U2200 NoSymbol
      ! exists
      keycode  26 = e E e E U2203 NoSymbol U2203 NoSymbol

  and then run xmodmap /.xmodmaprc.

Alternatively, you may use an input method editor such as SCIM or iBus. The latter offers a LATEX-like input method.

#### 154 How to customize the shortcuts for menus?

Two solutions are offered:

- Edit `$XDG_CONFIG_HOME/coq/coqide.keys` (which is usually `$HOME/.config/coq/coqide.keys`) by hand or
- If your system supports it, from CoqIde, you may select a menu entry and press the desired shortcut.

#### 155 What encoding should I use? What is this \x{iiii} in my file?

The encoding option is related to the way files are saved. Keep it as UTF-8 until it becomes important for you to exchange files with non UTF-8 aware applications. If you choose something else than UTF-8, then missing characters will be encoded by \x{....} or \x{........} where each dot is an hex. digit. The number between braces is the hexadecimal UNICODE index for the missing character.

#### 156 How to get rid of annoying unwanted automatic templates?

Some users may experiment problems with unwanted automatic templates while using Coqide. This is due to a change in the modifiers keys available through GTK. The straightest way to get rid of the problem is to edit by hand your coqiderc (either `/home/<user>/.config/coq/coqiderc` under Linux, or
`C:\Documents and Settings\<user>\.config\coq\coqiderc` under Windows) and replace any occurrence of MOD4 by MOD1.

## 15 Extraction

#### 157 What is program extraction?

Program extraction consist in generating a program from a constructive proof.

#### 158 Which language can I extract to?

You can extract your programs to Objective Caml and Haskell.

#### 159 How can I extract an incomplete proof?

You can provide programs for your axioms.

## 16 Glossary

#### 160 Can you explain me what an evaluable constant is?

An evaluable constant is a constant which is unfoldable.

#### 161 What is a goal?

The goal is the statement to be proved.

#### 162 What is a meta variable?

A meta variable in Coq represents a “hole”, i.e. a part of a proof that is still unknown.

#### 163 What is Gallina?

Gallina is the specification language of Coq. Complete documentation of this language can be found in the Reference Manual.

#### 164 What is The Vernacular?

It is the language of commands of Gallina i.e. definitions, lemmas, …

#### 165 What is a dependent type?

A dependent type is a type which depends on some term. For instance “vector of size n” is a dependent type representing all the vectors of size n. Its type depends on n

#### 166 What is a proof by reflection?

This is a proof generated by some computation which is done using the internal reduction of Coq (not using the tactic language of Coq (Ltac) nor the implementation language for Coq). An example of tactic using the reflection mechanism is the ring tactic. The reflection method consist in reflecting a subset of Coq language (for example the arithmetical expressions) into an object of the Coq language itself (in this case an inductive type denoting arithmetical expressions). For more information see [14, 12, 3] and the last chapter of the Coq’Art.

#### 167 What is intuitionistic logic?

This is any logic which does not assume that “A or not A”.

#### 168 What is proof-irrelevance?

See question 33

#### 169 What is the difference between opaque and transparent?

Opaque definitions can not be unfolded but transparent ones can.

## 17 Troubleshooting

#### 170 What can I do when Qed. is slow?

Sometime you can use the abstract tactic, which makes as if you had stated some local lemma, this speeds up the typing process.

#### 171 Why Reset Initial. does not work when using coqc?

The initial state corresponds to the state of coqtop when the interactive session began. It does not make sense in files to compile.

#### 172 What can I do if I get “No more subgoals but non-instantiated existential variables”?

This means that eauto or eapply didn’t instantiate an existential variable which eventually got erased by some computation. You may backtrack to the faulty occurrence of eauto or eapply and give the missing argument an explicit value. Alternatively, you can use the commands Show Existentials. and Existential. to display and instantiate the remaining existential variables.

Coq < Lemma example_show_existentials : forall a b c:nat, a=b -> b=c -> a=c.
1 subgoal
============================
forall a b c : nat, a = b -> b = c -> a = c
Coq < Proof.
1 subgoal
============================
forall a b c : nat, a = b -> b = c -> a = c
Coq < intros.
1 subgoal
a, b, c : nat
H : a = b
H0 : b = c
============================
a = c
Coq < eapply eq_trans.
2 focused subgoals
(shelved: 1)
a, b, c : nat
H : a = b
H0 : b = c
============================
a = ?y
subgoal 2 is:
?y = c
Coq < Show Existentials.
Existential 1 =
?y : [a : nat b : nat c : nat H : a = b H0 : b = c |- nat]
Existential 2 =
?Goal : [a : nat b : nat c : nat H : a = b H0 : b = c |- a = ?y]
Existential 3 =
?Goal0 : [a : nat b : nat c : nat H : a = b H0 : b = c |- ?y = c]
Coq < eassumption.
1 subgoal
a, b, c : nat
H : a = b
H0 : b = c
============================
b = c
Coq < assumption.
No more subgoals.
Coq < Qed.

#### 173 What can I do if I get “Cannot solve a second-order unification problem”?

You can help Coq using the pattern tactic.

#### 174 I copy-paste a term and Coq says it is not convertible to the original term. Sometimes it even says the copied term is not well-typed.

This is probably due to invisible implicit information (implicit arguments, coercions and Cases annotations) in the printed term, which is not re-synthesised from the copied-pasted term in the same way as it is in the original term.

Consider for instance (@eq Type True True). This term is printed as True=True and re-parsed as (@eq Prop True True). The two terms are not convertible (hence they fool tactics like pattern).

There is currently no satisfactory answer to the problem. However, the command Set Printing All is useful for diagnosing the problem.

Due to coercions, one may even face type-checking errors. In some rare cases, the criterion to hide coercions is a bit too loose, which may result in a typing error message if the parser is not able to find again the missing coercion.

## 18 Conclusion and Farewell.

#### 175 What if my question isn’t answered here?

Don’t panic `:-)`. You can try the Coq manual [17] for a technical description of the prover. The Coq’Art [2] is the first book written on Coq and provides a comprehensive review of the theorem prover as well as a number of example and exercises. Finally, the tutorial [15] provides a smooth introduction to theorem proving in Coq.

## References

[1]
  Yves bertot and Pierre Castéran. Coq’Art. Springer-Verlag, 2004. To appear.
[2]
  Yves Bertot and Pierre Castéran. Interactive Theorem Proving and Program Development, Coq’Art: The Calculus of Inductive Constructions. Texts in Theoretical Computer Science. An EATCS series. Springer Verlag, 2004.
[3]
  Samuel Boutin. Using reflection to build efficient and certified decision pro cedures. In M. Abadi and T. Ito, editors, Proceedings of TACS’97, volume 1281 of LNCS. Springer-Verlag, 1997.
[4]
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[5]
  Thierry Coquand. Une Théorie des Constructions. PhD thesis, Université Paris 7, January 1985.
[6]
  Thierry Coquand and Gérard Huet. The Calculus of Constructions. Information and Computation, 76(2/3), 1988.
[7]
  Thierry Coquand and Christine Paulin-Mohring. Inductively defined types. In P. Martin-Löf and G. Mints, editors, Proceedings of Colog’88, volume 417 of Lecture Notes in Computer Science. Springer-Verlag, 1990.
[8]
  Gilles Dowek. Théorie des types. Lecture notes, 2002.
[9]
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[10]
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[11]
  Jean-Yves Girard, Yves Lafont, and Paul Taylor. Proofs and Types. Cambrige Tracts in Theoretical Computer Science, Cambridge University Press, 1989.
[12]
  John Harrison. Meta theory and reflection in theorem proving:a survey and cri tique. Technical Report CRC-053, SRI International Cambridge Computer Science Research Center, 1995.
[13]
  Martin Hofmann and Thomas Streicher. The groupoid interpretation of type theory. In Proceedings of the meeting Twenty-five years of constructive type theory. Oxford University Press, 1998.
[14]
  Doug Howe. Computation meta theory in nuprl. In E. Lusk and R. Overbeek, editors, The Proceedings of the Ninth International Conference of Autom ated Deduction, volume 310, pages 238–257. Springer-Verlag, 1988.
[15]
  Gérard Huet, Gilles Kahn, and Christine Paulin-Mohring. The Coq Proof Assistant A Tutorial, 2004.
[16]
  Z. Luo. An Extended Calculus of Constructions. PhD thesis, University of Edinburgh, 1990.
[17]
  The Coq development team. The Coq proof assistant reference manual. LogiCal Project, 2004. Version 8.0.
[18]
  Tobias Oetiker. The Not So Short Introduction to LATEX2e, January 1999.
[19]
  Christine Paulin-Mohring. Définitions Inductives en Théorie des Types d’Ordre Supérieur. Habilitation à diriger les recherches, Université Claude Bernard Lyon I, December 1996.

> This document was translated from LATEX by HEVEA.