Generalized rewriting — Coq 8.8.1 documentation

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# Generalized rewriting

Author: | Matthieu Sozeau

This chapter presents the extension of several equality related tactics to work over user-defined structures (called setoids) that are equipped with ad-hoc equivalence relations meant to behave as equalities. Actually, the tactics have also been generalized to relations weaker then equivalences (e.g. rewriting systems). The toolbox also extends the automatic rewriting capabilities of the system, allowing the specification of custom strategies for rewriting.

This documentation is adapted from the previous setoid documentation by Claudio Sacerdoti Coen (based on previous work by Clément Renard). The new implementation is a drop-in replacement for the old one [1], hence most of the documentation still applies.

The work is a complete rewrite of the previous implementation, based on the type class infrastructure. It also improves on and generalizes the previous implementation in several ways:

- User-extensible algorithm. The algorithm is separated in two parts: generations of the rewriting constraints (done in ML) and solving of these constraints using type class resolution. As type class resolution is extensible using tactics, this allows users to define general ways to solve morphism constraints.
- Sub-relations. An example extension to the base algorithm is the ability to define one relation as a subrelation of another so that morphism declarations on one relation can be used automatically for the other. This is done purely using tactics and type class search.
- Rewriting under binders. It is possible to rewrite under binders in the new implementation, if one provides the proper morphisms. Again, most of the work is handled in the tactics.
- First-class morphisms and signatures. Signatures and morphisms are ordinary Coq terms, hence they can be manipulated inside Coq, put inside structures and lemmas about them can be proved inside the system. Higher-order morphisms are also allowed.
- Performance. The implementation is based on a depth-first search for the first solution to a set of constraints which can be as fast as linear in the size of the term, and the size of the proof term is linear in the size of the original term. Besides, the extensibility allows the user to customize the proof search if necessary.

[1] | Nicolas Tabareau helped with the gluing.

## Introduction to generalized rewriting

### Relations and morphisms

A parametric relation `R` is any term of type `forall (x1 :T1 ) ... (xn :Tn ), relation A`. The expression `A`, which depends on `x1 ... xn` , is called the carrier of the relation and `R` is said to be a relation over `A`; the list `x1,...,xn` is the (possibly empty) list of parameters of the relation.

Example 1 (Parametric relation):

It is possible to implement finite sets of elements of type `A` as unordered list of elements of type `A`. The function `set_eq: forall (A: Type), relation (list A)` satisfied by two lists with the same elements is a parametric relation over `(list A)` with one parameter `A`. The type of `set_eq` is convertible with `forall (A: Type), list A -> list A -> Prop.`

An instance of a parametric relation `R` with n parameters is any term `(R t1 ... tn )`.

Let `R` be a relation over `A` with `n` parameters. A term is a parametric proof of reflexivity for `R` if it has type `forall (x1 :T1 ) ... (xn :Tn), reflexive (R x1 ... xn )`. Similar definitions are given for parametric proofs of symmetry and transitivity.

Example 2 (Parametric relation (cont.)):

The `set_eq` relation of the previous example can be proved to be reflexive, symmetric and transitive. A parametric unary function `f` of type `forall (x1 :T1 ) ... (xn :Tn ), A1 -> A2` covariantly respects two parametric relation instances `R1` and `R2` if, whenever `x`, `y` satisfy `R1 x y`, their images (`f x`) and (`f y`) satisfy `R2 (f x) (f y)`. An `f` that respects its input and output relations will be called a unary covariant morphism. We can also say that `f` is a monotone function with respect to `R1` and `R2` . The sequence `x1 ... xn` represents the parameters of the morphism.

Let `R1` and `R2` be two parametric relations. The signature of a parametric morphism of type `forall (x1 :T1 ) ... (xn :Tn ), A1 -> A2` that covariantly respects two instances \(I_{R_1}\) and \(I_{R_2}\) of `R1` and `R2` is written \(I_{R_1} ++> I_{R_2}\). Notice that the special arrow ++>, which reminds the reader of covariance, is placed between the two relation instances, not between the two carriers. The signature relation instances and morphism will be typed in a context introducing variables for the parameters.

The previous definitions are extended straightforwardly to n-ary morphisms, that are required to be simultaneously monotone on every argument.

Morphisms can also be contravariant in one or more of their arguments. A morphism is contravariant on an argument associated to the relation instance \(R\) if it is covariant on the same argument when the inverse relation \(R^{−1}\) (`inverse R` in Coq) is considered. The special arrow `-->` is used in signatures for contravariant morphisms.

Functions having arguments related by symmetric relations instances are both covariant and contravariant in those arguments. The special arrow `==>` is used in signatures for morphisms that are both covariant and contravariant.

An instance of a parametric morphism \(f\) with \(n\) parameters is any term \(f \, t_1 \ldots t_n\).

Example 3 (Morphisms):

Continuing the previous example, let `union: forall (A: Type), list A -> list A -> list A` perform the union of two sets by appending one list to the other. `union` is a binary morphism parametric over ``A` that respects the relation instance `(set_eq A)`. The latter condition is proved by showing:

forall (A: Type) (S1 S1’ S2 S2’: list A), set_eq A S1 S1’ -> set_eq A S2 S2’ -> set_eq A (union A S1 S2) (union A S1’ S2’).
  Toplevel input, characters 0-6: > forall (A: Type) (S1 S1’ S2 S2’: list A), set_eq A S1 S1’ -> set_eq A S2 S2’ -> set_eq A (union A S1 S2) (union A S1’ S2’). > ^^^^^^ Error: Syntax error: illegal begin of vernac.

The signature of the function `union A` is `set_eq A ==> set_eq A ==> set_eq A` for all `A`.

Example 4 (Contravariant morphism):

The division function `Rdiv: R -> R -> R` is a morphism of signature `le ++> le --> le` where `le` is the usual order relation over real numbers. Notice that division is covariant in its first argument and contravariant in its second argument.

Leibniz equality is a relation and every function is a morphism that respects Leibniz equality. Unfortunately, Leibniz equality is not always the intended equality for a given structure.

In the next section we will describe the commands to register terms as parametric relations and morphisms. Several tactics that deal with equality in Coq can also work with the registered relations. The exact list of tactic will be given in this section. For instance, the tactic reflexivity can be used to close a goal `R n n` whenever `R` is an instance of a registered reflexive relation. However, the tactics that replace in a context `C[]` one term with another one related by `R` must verify that `C[]` is a morphism that respects the intended relation. Currently the verification consists in checking whether `C[]` is a syntactic composition of morphism instances that respects some obvious compatibility constraints.

Example 5 (Rewriting):

Continuing the previous examples, suppose that the user must prove `set_eq int (union int (union int S1 S2) S2) (f S1 S2)` under the hypothesis `H: set_eq int S2 (@nil int)`. It is possible to use the `rewrite` tactic to replace the first two occurrences of `S2` with `@nil int` in the goal since the context `set_eq int (union int (union int S1 nil) nil) (f S1 S2)`, being a composition of morphisms instances, is a morphism. However the tactic will fail replacing the third occurrence of `S2` unless `f` has also been declared as a morphism.

### Adding new relations and morphisms

A parametric relation `Aeq: forall (y1 : β1 ... ym : βm )`, `relation (A t1 ... tn)` over `(A : αi -> ... αn -> Type)` can be declared with the following command:

Command `Add Parametric Relation (x1 : T1) ... (xn : Tk) : (A t1 ... tn) (Aeq t′1 ... t′m ) reflexivity proved by refl? symmetry proved by sym? transitivity proved by trans? as ident`

after having required the `Setoid` module with the `Require Setoid` command.

The `@ident` gives a unique name to the morphism and it is used by the command to generate fresh names for automatically provided lemmas used internally.

Notice that the carrier and relation parameters may refer to the context of variables introduced at the beginning of the declaration, but the instances need not be made only of variables. Also notice that `A` is not required to be a term having the same parameters as `Aeq`, although that is often the case in practice (this departs from the previous implementation).

Command `Add Relation`

In case the carrier and relations are not parametric, one can use this command instead, whose syntax is the same except there is no local context.

The proofs of reflexivity, symmetry and transitivity can be omitted if the relation is not an equivalence relation. The proofs must be instances of the corresponding relation definitions: e.g. the proof of reflexivity must have a type convertible to `reflexive (A t1 ... tn) (Aeq t′ 1 …t′ n )`. Each proof may refer to the introduced variables as well.

Example 6 (Parametric relation):

For Leibniz equality, we may declare:

Add Parametric Relation (A : Type) : A (@eq A) [reflexivity proved by @refl_equal A] ...
  Toplevel input, characters 49-50: > Add Parametric Relation (A : Type) : A (@eq A) [reflexivity proved by @refl_equal A] ... > ^ Error: Syntax error: 'transitivity' 'proved' 'by' expected after [constr:constr] (in [vernac:command]).

Some tactics (`reflexivity`, `symmetry`, `transitivity`) work only on relations that respect the expected properties. The remaining tactics (replace, `rewrite` and derived tactics such as `autorewrite`) do not require any properties over the relation. However, they are able to replace terms with related ones only in contexts that are syntactic compositions of parametric morphism instances declared with the following command.

Command `Add Parametric Morphism (x1 : T1 ) ... (xk : Tk ) : (f t1 ... tn ) with signature sig as ident`

The command declares `f` as a parametric morphism of signature `sig`. The identifier `id` gives a unique name to the morphism and it is used as the base name of the type class instance definition and as the name of the lemma that proves the well-definedness of the morphism. The parameters of the morphism as well as the signature may refer to the context of variables. The command asks the user to prove interactively that `f` respects the relations identified from the signature.

Example 7:

We start the example by assuming a small theory over homogeneous sets and we declare set equality as a parametric equivalence relation and union of two sets as a parametric morphism.

Require Export Setoid.
Require Export Relation_Definitions.
Set Implicit Arguments.
Parameter set: Type -> Type.
  set is declared
Parameter empty: forall A, set A.
  empty is declared
Parameter eq_set: forall A, set A -> set A -> Prop.
  eq_set is declared
Parameter union: forall A, set A -> set A -> set A.
  union is declared
Axiom eq_set_refl: forall A, reflexive _ (eq_set (A:=A)).
  eq_set_refl is declared
Axiom eq_set_sym: forall A, symmetric _ (eq_set (A:=A)).
  eq_set_sym is declared
Axiom eq_set_trans: forall A, transitive _ (eq_set (A:=A)).
  eq_set_trans is declared
Axiom empty_neutral: forall A (S: set A), eq_set (union S (empty A)) S.
  empty_neutral is declared
Axiom union_compat: forall (A : Type), forall x x' : set A, eq_set x x' -> forall y y' : set A, eq_set y y' -> eq_set (union x y) (union x' y').
  union_compat is declared
Add Parametric Relation A : (set A) (@eq_set A) reflexivity proved by (eq_set_refl (A:=A)) symmetry proved by (eq_set_sym (A:=A)) transitivity proved by (eq_set_trans (A:=A)) as eq_set_rel.
  eq_set_rel_relation is defined eq_set_rel_Reflexive is defined eq_set_rel_Symmetric is defined eq_set_rel_Transitive is defined eq_set_rel is defined
Add Parametric Morphism A : (@union A) with signature (@eq_set A) ==> (@eq_set A) ==> (@eq_set A) as union_mor.
  1 subgoal A : Type ============================ forall x y : set A, eq_set x y -> forall x0 y0 : set A, eq_set x0 y0 -> eq_set (union x x0) (union y y0)
Proof.
exact (@union_compat A).
  No more subgoals.
Qed.
  union_mor_Proper is defined

It is possible to reduce the burden of specifying parameters using (maximally inserted) implicit arguments. If `A` is always set as maximally implicit in the previous example, one can write:

Add Parametric Relation A : (set A) eq_set reflexivity proved by eq_set_refl symmetry proved by eq_set_sym transitivity proved by eq_set_trans as eq_set_rel.
  Toplevel input, characters 36-42: > Add Parametric Relation A : (set A) eq_set reflexivity proved by eq_set_refl symmetry proved by eq_set_sym transitivity proved by eq_set_trans as eq_set_rel. > ^^^^^^ Error: In environment A : Type The term "eq_set" has type "forall A : Type, set A -> set A -> Prop" while it is expected to have type "relation (set A)" (cannot unify "set A" and "Type").

Add Parametric Morphism A : (@union A) with signature eq_set ==> eq_set ==> eq_set as union_mor.
  Toplevel input, characters 56-62: > Add Parametric Morphism A : (@union A) with signature eq_set ==> eq_set ==> eq_set as union_mor. > ^^^^^^ Error: In environment A : ?T The term "eq_set" has type "forall A : Type, set A -> set A -> Prop" while it is expected to have type "relation Type" (cannot unify "Type" and "set A0").

Proof.
exact (@union_compat A).
  Toplevel input, characters 0-24: > exact (@union_compat A). > ^^^^^^^^^^^^^^^^^^^^^^^^ Error: No focused proof (No proof-editing in progress).
Qed.
  Toplevel input, characters 0-4: > Qed. > ^^^^ Error: No focused proof (No proof-editing in progress).

We proceed now by proving a simple lemma performing a rewrite step and then applying reflexivity, as we would do working with Leibniz equality. Both tactic applications are accepted since the required properties over `eq_set` and `union` can be established from the two declarations above.

Goal forall (S: set nat), eq_set (union (union S empty) S) (union S S).
  Toplevel input, characters 51-56: > Goal forall (S: set nat), eq_set (union (union S empty) S) (union S S). > ^^^^^ Error: In environment S : set nat The term "empty" has type "forall A : Type, set A" while it is expected to have type "set nat".

Proof.
intros.
  Toplevel input, characters 0-7: > intros. > ^^^^^^^ Error: No focused proof (No proof-editing in progress).
rewrite empty_neutral.
  Toplevel input, characters 0-22: > rewrite empty_neutral. > ^^^^^^^^^^^^^^^^^^^^^^ Error: No focused proof (No proof-editing in progress).
reflexivity.
  Toplevel input, characters 0-12: > reflexivity. > ^^^^^^^^^^^^ Error: No focused proof (No proof-editing in progress).
Qed.
  Toplevel input, characters 0-4: > Qed. > ^^^^ Error: No focused proof (No proof-editing in progress).

The tables of relations and morphisms are managed by the type class instance mechanism. The behavior on section close is to generalize the instances by the variables of the section (and possibly hypotheses used in the proofs of instance declarations) but not to export them in the rest of the development for proof search. One can use the cmd:Existing Instance command to do so outside the section, using the name of the declared morphism suffixed by `_Morphism`, or use the `Global` modifier for the corresponding class instance declaration (see First Class Setoids and Morphisms) at definition time. When loading a compiled file or importing a module, all the declarations of this module will be loaded.

### Rewriting and non reflexive relations

To replace only one argument of an n-ary morphism it is necessary to prove that all the other arguments are related to themselves by the respective relation instances.

Example 8:

To replace `(union S empty)` with `S` in `(union (union S empty) S) (union S S)` the rewrite tactic must exploit the monotony of `union` (axiom `union_compat` in the previous example). Applying `union_compat` by hand we are left with the goal `eq_set (union S S) (union S S)`.

When the relations associated to some arguments are not reflexive, the tactic cannot automatically prove the reflexivity goals, that are left to the user.

Setoids whose relation are partial equivalence relations (PER) are useful to deal with partial functions. Let `R` be a PER. We say that an element `x` is defined if `R x x`. A partial function whose domain comprises all the defined elements only is declared as a morphism that respects `R`. Every time a rewriting step is performed the user must prove that the argument of the morphism is defined.

Example 9:

Let `eqO` be `fun x y => x = y /\ x <> 0` (the smaller PER over non zero elements). Division can be declared as a morphism of signature `eq ==> eq0 ==> eq`. Replace `x` with `y` in `div x n = div y n` opens the additional goal `eq0 n n` that is equivalent to `n = n /\ n <> 0`.

### Rewriting and non symmetric relations

When the user works up to relations that are not symmetric, it is no longer the case that any covariant morphism argument is also contravariant. As a result it is no longer possible to replace a term with a related one in every context, since the obtained goal implies the previous one if and only if the replacement has been performed in a contravariant position. In a similar way, replacement in an hypothesis can be performed only if the replaced term occurs in a covariant position.

Example 10 (Covariance and contravariance):

Suppose that division over real numbers has been defined as a morphism of signature `Z.div: Z.lt ++> Z.lt --> Z.lt` (i.e. `Z.div` is increasing in its first argument, but decreasing on the second one). Let `<` denotes `Z.lt`. Under the hypothesis `H: x < y` we have `k < x / y -> k < x / x`, but not `k < y / x -> k < x / x`. Dually, under the same hypothesis `k < x / y -> k < y / y` holds, but `k < y / x -> k < y / y` does not. Thus, if the current goal is `k < x / x`, it is possible to replace only the second occurrence of `x` (in contravariant position) with `y` since the obtained goal must imply the current one. On the contrary, if `k < x / x` is an hypothesis, it is possible to replace only the first occurrence of `x` (in covariant position) with `y` since the current hypothesis must imply the obtained one.

Contrary to the previous implementation, no specific error message will be raised when trying to replace a term that occurs in the wrong position. It will only fail because the rewriting constraints are not satisfiable. However it is possible to use the at modifier to specify which occurrences should be rewritten.

As expected, composing morphisms together propagates the variance annotations by switching the variance every time a contravariant position is traversed.

Example 11:

Let us continue the previous example and let us consider the goal `x / (x / x) < k`. The first and third occurrences of `x` are in a contravariant position, while the second one is in covariant position. More in detail, the second occurrence of `x` occurs covariantly in `(x / x)` (since division is covariant in its first argument), and thus contravariantly in `x / (x / x)` (since division is contravariant in its second argument), and finally covariantly in `x / (x / x) < k` (since `<`, as every transitive relation, is contravariant in its first argument with respect to the relation itself).

### Rewriting in ambiguous setoid contexts

One function can respect several different relations and thus it can be declared as a morphism having multiple signatures.

Example 12:

Union over homogeneous lists can be given all the following signatures: `eq ==> eq ==> eq` (`eq` being the equality over ordered lists) `set_eq ==> set_eq ==> set_eq` (`set_eq` being the equality over unordered lists up to duplicates), `multiset_eq ==> multiset_eq ==> multiset_eq` (`multiset_eq` being the equality over unordered lists).

To declare multiple signatures for a morphism, repeat the `Add Morphism` command.

When morphisms have multiple signatures it can be the case that a rewrite request is ambiguous, since it is unclear what relations should be used to perform the rewriting. Contrary to the previous implementation, the tactic will always choose the first possible solution to the set of constraints generated by a rewrite and will not try to find all possible solutions to warn the user about.

## Commands and tactics

### First class setoids and morphisms

The implementation is based on a first-class representation of properties of relations and morphisms as type classes. That is, the various combinations of properties on relations and morphisms are represented as records and instances of theses classes are put in a hint database. For example, the declaration:

Add Parametric Relation (x1 : T1) ... (xn : Tk) : (A t1 ... tn) (Aeq t′1 ... t′m) [reflexivity proved by refl] [symmetry proved by sym] [transitivity proved by trans] as id.
  Toplevel input, characters 34-37: > Add Parametric Relation (x1 : T1) ... > ^^^ Error: Syntax error: ':' expected after [tactic:binders] (in [vernac:command]). Toplevel input, characters 42-43: > Add Parametric Relation (x1 : T1) ... (xn : Tk) : (A t1 ... > ^ Error: Syntax error: ')' expected after [tactic:tactic_expr] (in [tactic:tactic_expr]). Toplevel input, characters 62-63: > Add Parametric Relation (x1 : T1) ... (xn : Tk) : (A t1 ... tn) (Aeq t′1 ... > ^ Error: Syntax error: [tactic:ltac_use_default] expected after [tactic:tactic] (in [vernac:tactic_command]). Toplevel input, characters 80-83: > Add Parametric Relation (x1 : T1) ... (xn : Tk) : (A t1 ... tn) (Aeq t′1 ... t′m) [reflexivity proved by refl] [symmetry proved by sym] [transitivity proved by trans] as id. > ^ Error: Syntax Error: Lexer: Undefined token

is equivalent to an instance declaration:

Instance (x1 : T1) ... (xn : Tk) => id : @Equivalence (A t1 ... tn) (Aeq t′1 ... t′m) := [Equivalence_Reflexive := refl] [Equivalence_Symmetric := sym] [Equivalence_Transitive := trans].
  Toplevel input, characters 9-10: > Instance (x1 : T1) ... > ^ Error: Syntax error: [vernac:instance_name] expected after 'Instance' (in [vernac:gallina_ext]). Toplevel input, characters 27-28: > Instance (x1 : T1) ... (xn : Tk) => id : @Equivalence (A t1 ... > ^ Error: Syntax error: ')' expected after [tactic:tactic_expr] (in [tactic:tactic_expr]). Toplevel input, characters 66-67: > Instance (x1 : T1) ... (xn : Tk) => id : @Equivalence (A t1 ... tn) (Aeq t′1 ... > ^ Error: Syntax error: [tactic:ltac_use_default] expected after [tactic:tactic] (in [vernac:tactic_command]). Toplevel input, characters 84-87: > Instance (x1 : T1) ... (xn : Tk) => id : @Equivalence (A t1 ... tn) (Aeq t′1 ... t′m) := [Equivalence_Reflexive := refl] [Equivalence_Symmetric := sym] [Equivalence_Transitive := trans]. > ^ Error: Syntax Error: Lexer: Undefined token

The declaration itself amounts to the definition of an object of the record type `Coq.Classes.RelationClasses.Equivalence` and a hint added to the `typeclass_instances` hint database. Morphism declarations are also instances of a type class defined in `Classes.Morphisms`. See the documentation on type classes Type Classes and the theories files in Classes for further explanations.

One can inform the rewrite tactic about morphisms and relations just by using the typeclass mechanism to declare them using Instance and Context vernacular commands. Any object of type Proper (the type of morphism declarations) in the local context will also be automatically used by the rewriting tactic to solve constraints.

Other representations of first class setoids and morphisms can also be handled by encoding them as records. In the following example, the projections of the setoid relation and of the morphism function can be registered as parametric relations and morphisms.

Example 13 (First class setoids):

Require Import Relation_Definitions Setoid.
Record Setoid: Type := { car: Type; eq: car -> car -> Prop; refl: reflexive _ eq; sym: symmetric _ eq; trans: transitive _ eq }.
  Setoid is defined car is defined eq is defined refl is defined sym is defined trans is defined
Add Parametric Relation (s : Setoid) : (@car s) (@eq s) reflexivity proved by (refl s) symmetry proved by (sym s) transitivity proved by (trans s) as eq_rel.
  eq_rel_relation is defined eq_rel_Reflexive is defined eq_rel_Symmetric is defined eq_rel_Transitive is defined eq_rel is defined
Record Morphism (S1 S2: Setoid): Type := { f: car S1 -> car S2; compat: forall (x1 x2: car S1), eq S1 x1 x2 -> eq S2 (f x1) (f x2) }.
  Morphism is defined f is defined compat is defined
Add Parametric Morphism (S1 S2 : Setoid) (M : Morphism S1 S2) : (@f S1 S2 M) with signature (@eq S1 ==> @eq S2) as apply_mor.
  1 subgoal S1, S2 : Setoid M : Morphism S1 S2 ============================ forall x y : car S1, eq S1 x y -> eq S2 (f M x) (f M y)
Proof.
apply (compat S1 S2 M).
  Toplevel input, characters 14-16: > apply (compat S1 S2 M). > ^^ Error: In environment S1, S2 : Setoid M : Morphism S1 S2 The term "S1" has type "Setoid" while it is expected to have type "Morphism ?S1 ?S2".
Qed.
  Toplevel input, characters 0-4: > Qed. > ^^^^ Error: Attempt to save an incomplete proof (in proof apply_mor_Proper)
Lemma test: forall (S1 S2: Setoid) (m: Morphism S1 S2) (x y: car S1), eq S1 x y -> eq S2 (f _ _ m x) (f _ _ m y).
  Toplevel input, characters 91-98: > Lemma test: forall (S1 S2:Setoid) (m: Morphism S1 S2) (x y: car S1), eq S1 x y -> eq S2 (f _ _ m x) (f _ _ m y). > ^^^^^^^ Error: Illegal application (Non-functional construction): The expression "f ?m ?c" of type "car ?S2" cannot be applied to the term "m" : "Morphism S1 S2"
Proof.
intros.
  1 subgoal S1, S2 : Setoid M : Morphism S1 S2 x, y : car S1 H : eq S1 x y ============================ eq S2 (f M x) (f M y)
rewrite H.
  Toplevel input, characters 0-9: > rewrite H. > ^^^^^^^^^ Error: Tactic failure: setoid rewrite failed: Unable to satisfy the following constraints: UNDEFINED EVARS: ?X102==[S1 S2 M x y H |- relation (car S2)] (internal placeholder) {?r} ?X103==[S1 S2 M x y H (do_subrelation:=Morphisms.do_subrelation) |- Morphisms.Proper (eq S1 ==> ?X102@{__:=S1; __:=S2; __:=M; __:=x; __:=y; __:=H}) (f M)] (internal placeholder) {?p} ?X105==[S1 S2 M x y H |- relation (car S2)] (internal placeholder) {?r0} ?X106==[S1 S2 M x y H (do_subrelation:=Morphisms.do_subrelation) |- Morphisms.Proper (?X102@{__:=S1; __:=S2; __:=M; __:=x; __:=y; __:=H} ==> ?X105@{__:=S1; __:=S2; __:=M; __:=x; __:=y; __:=H} ==> Basics.flip Basics.impl) (eq S2)] (internal placeholder) {?p0} ?X107==[S1 S2 M x y H |- Morphisms.ProperProxy ?X105@{__:=S1; __:=S2; __:=M; __:=x; __:=y; __:=H} (f M y)] (internal placeholder) {?p1}
reflexivity.
  Toplevel input, characters 0-11: > reflexivity. > ^^^^^^^^^^^ Error: In environment S1, S2 : Setoid M : Morphism S1 S2 x, y : car S1 H : eq S1 x y Unable to unify "f M y" with "f M x".
Qed.
  Toplevel input, characters 0-4: > Qed. > ^^^^ Error: Attempt to save an incomplete proof (in proof apply_mor_Proper)

### Tactics enabled on user provided relations

The following tactics, all prefixed by `setoid_`, deal with arbitrary registered relations and morphisms. Moreover, all the corresponding unprefixed tactics (i.e. `reflexivity`, `symmetry`, `transitivity`, `replace`, `rewrite`) have been extended to fall back to their prefixed counterparts when the relation involved is not Leibniz equality. Notice, however, that using the prefixed tactics it is possible to pass additional arguments such as `using relation`.

Variant `setoid_reflexivity`

Variant `setoid_symmetry [in ident]`

Variant `setoid_transitivity`

Variant `setoid_rewrite [orientation] term [at occs] [in ident]`

Variant `setoid_replace term with term [in ident] [using relation term] [by tactic]`

The `using relation` arguments cannot be passed to the unprefixed form. The latter argument tells the tactic what parametric relation should be used to replace the first tactic argument with the second one. If omitted, it defaults to the `DefaultRelation` instance on the type of the objects. By default, it means the most recent `Equivalence` instance in the environment, but it can be customized by declaring new `DefaultRelation` instances. As Leibniz equality is a declared equivalence, it will fall back to it if no other relation is declared on a given type.

Every derived tactic that is based on the unprefixed forms of the tactics considered above will also work up to user defined relations. For instance, it is possible to register hints for `autorewrite` that are not proof of Leibniz equalities. In particular it is possible to exploit `autorewrite` to simulate normalization in a term rewriting system up to user defined equalities.

### Printing relations and morphisms

Command `Print Instances`

This command can be used to show the list of currently registered `Reflexive` (using `Print Instances Reflexive`), `Symmetric` or `Transitive` relations, Equivalences, PreOrders, PERs, and Morphisms (implemented as `Proper` instances). When the rewriting tactics refuse to replace a term in a context because the latter is not a composition of morphisms, the `Print Instances` command can be useful to understand what additional morphisms should be registered.

### Deprecated syntax and backward incompatibilities

Due to backward compatibility reasons, the following syntax for the declaration of setoids and morphisms is also accepted.

Command `Add Setoid A Aeq ST as ident`

where `Aeq` is a congruence relation without parameters, `A` is its carrier and `ST` is an object of type (`Setoid_Theory A Aeq`) (i.e. a record packing together the reflexivity, symmetry and transitivity lemmas). Notice that the syntax is not completely backward compatible since the identifier was not required.

Command `Add Morphism f : ident`

The latter command also is restricted to the declaration of morphisms without parameters. It is not fully backward compatible since the property the user is asked to prove is slightly different: for n-ary morphisms the hypotheses of the property are permuted; moreover, when the morphism returns a proposition, the property is now stated using a bi-implication in place of a simple implication. In practice, porting an old development to the new semantics is usually quite simple.

Notice that several limitations of the old implementation have been lifted. In particular, it is now possible to declare several relations with the same carrier and several signatures for the same morphism. Moreover, it is now also possible to declare several morphisms having the same signature. Finally, the `replace` and `rewrite` tactics can be used to replace terms in contexts that were refused by the old implementation. As discussed in the next section, the semantics of the new `setoid_rewrite` tactic differs slightly from the old one and `rewrite`.

## Extensions

### Rewriting under binders

Warning

Due to compatibility issues, this feature is enabled only when calling the `setoid_rewrite` tactic directly and not `rewrite`.

To be able to rewrite under binding constructs, one must declare morphisms with respect to pointwise (setoid) equivalence of functions. Example of such morphisms are the standard `all` and `ex` combinators for universal and existential quantification respectively. They are declared as morphisms in the `Classes.Morphisms_Prop` module. For example, to declare that universal quantification is a morphism for logical equivalence:

Instance all_iff_morphism (A : Type) : Proper (pointwise_relation A iff ==> iff) (@all A).
  Toplevel input, characters 56-88: > Instance all_iff_morphism (A : Type) : Proper (pointwise_relation A iff ==> iff) (@all A). > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: Unknown interpretation for notation "_ ==> _".

Proof.
simpl_relation.
  1 subgoal S1, S2 : Setoid M : Morphism S1 S2 x, y : car S1 H : eq S1 x y ============================ (let (car0, eq0, _, _, _) as s return (car s -> car s -> Prop) := S2 in eq0) (f M x) (f M y)

One then has to show that if two predicates are equivalent at every point, their universal quantifications are equivalent. Once we have declared such a morphism, it will be used by the setoid rewriting tactic each time we try to rewrite under an `all` application (products in `Prop` are implicitly translated to such applications).

Indeed, when rewriting under a lambda, binding variable `x`, say from `P x` to `Q x` using the relation iff, the tactic will generate a proof of `pointwise_relation A iff (fun x => P x) (fun x => Q x)` from the proof of `iff (P x) (Q x)` and a constraint of the form Proper `(pointwise_relation A iff ==> ?) m` will be generated for the surrounding morphism `m`.

Hence, one can add higher-order combinators as morphisms by providing signatures using pointwise extension for the relations on the functional arguments (or whatever subrelation of the pointwise extension). For example, one could declare the `map` combinator on lists as a morphism:

Instance map_morphism `{Equivalence A eqA, Equivalence B eqB} : Proper ((eqA ==> eqB) ==> list_equiv eqA ==> list_equiv eqB) (@map A B).
  Toplevel input, characters 36-37: > Instance map_morphism `{Equivalence A eqA, Equivalence B eqB} : Proper ((eqA ==> eqB) ==> list_equiv eqA ==> list_equiv eqB) (@map A B). > ^ Error: Unbound and ungeneralizable variable A

where `list_equiv` implements an equivalence on lists parameterized by an equivalence on the elements.

Note that when one does rewriting with a lemma under a binder using `setoid_rewrite`, the application of the lemma may capture the bound variable, as the semantics are different from rewrite where the lemma is first matched on the whole term. With the new `setoid_rewrite`, matching is done on each subterm separately and in its local environment, and all matches are rewritten simultaneously by default. The semantics of the previous `setoid_rewrite` implementation can almost be recovered using the `at 1` modifier.

### Sub-relations

Sub-relations can be used to specify that one relation is included in another, so that morphisms signatures for one can be used for the other. If a signature mentions a relation `R` on the left of an arrow `==>`, then the signature also applies for any relation `S` that is smaller than `R`, and the inverse applies on the right of an arrow. One can then declare only a few morphisms instances that generate the complete set of signatures for a particular constant. By default, the only declared subrelation is `iff`, which is a subrelation of `impl` and `inverse impl` (the dual of implication). That’s why we can declare only two morphisms for conjunction: `Proper (impl ==> impl ==> impl) and` and `Proper (iff ==> iff ==> iff) and`. This is sufficient to satisfy any rewriting constraints arising from a rewrite using `iff`, `impl` or `inverse impl` through `and`.

Sub-relations are implemented in `Classes.Morphisms` and are a prime example of a mostly user-space extension of the algorithm.

### Constant unfolding

The resolution tactic is based on type classes and hence regards user- defined constants as transparent by default. This may slow down the resolution due to a lot of unifications (all the declared `Proper` instances are tried at each node of the search tree). To speed it up, declare your constant as rigid for proof search using the command `Typeclasses Opaque`.

## Strategies for rewriting

### Definitions

The generalized rewriting tactic is based on a set of strategies that can be combined to obtain custom rewriting procedures. Its set of strategies is based on Elan’s rewriting strategies [LV97]. Rewriting strategies are applied using the tactic `rewrite_strat s` where `s` is a strategy expression. Strategies are defined inductively as described by the following grammar:

    s, t, u ::=  strategy
                 | lemma
                 | lemma_right_to_left
                 | failure
                 | identity
                 | reflexivity
                 | progress
                 | failure_catch
                 | composition
                 | left_biased_choice
                 | iteration_one_or_more
                 | iteration_zero_or_more
                 | one_subterm
                 | all_subterms
                 | innermost_first
                 | outermost_first
                 | bottom_up
                 | top_down
                 | apply_hint
                 | any_of_the_terms
                 | apply_reduction
                 | fold_expression

    strategy               ::=  "(" s ")"
    lemma                  ::=  c
    lemma_right_to_left    ::=  "<-" c
    failure                ::=  fail
    identity               ::=  id
    reflexivity            ::=  refl
    progress               ::=  progress s
    failure_catch          ::=  try s
    composition            ::=  s ";" u
    left_biased_choice     ::=  choice s t
    iteration_one_or_more  ::=  repeat s
    iteration_zero_or_more ::=  any s
    one_subterm            ::=  subterm s
    all_subterms           ::=  subterms s
    innermost_first        ::=  innermost s
    outermost_first        ::=  outermost s
    bottom_up              ::=  bottomup s
    top_down               ::=  topdown s
    apply_hint             ::=  hints hintdb
    any_of_the_terms       ::=  terms (c)+
    apply_reduction        ::=  eval redexpr
    fold_expression        ::=  fold c

Actually a few of these are defined in term of the others using a primitive fixpoint operator:

    try `s`     ::=  choice s id
    any `s`     ::=  fix u. try (s ; u)
    repeat `s`  ::=  s ; any s
    bottomup s  ::=  fix bu. (choice (progress (subterms bu)) s) ; try bu
    topdown s   ::=  fix td. (choice s (progress (subterms td))) ; try td
    innermost s ::=  fix i. (choice (subterm i) s)
    outermost s ::=  fix o. (choice s (subterm o))

The basic control strategy semantics are straightforward: strategies are applied to subterms of the term to rewrite, starting from the root of the term. The lemma strategies unify the left-hand-side of the lemma with the current subterm and on success rewrite it to the right- hand-side. Composition can be used to continue rewriting on the current subterm. The fail strategy always fails while the identity strategy succeeds without making progress. The reflexivity strategy succeeds, making progress using a reflexivity proof of rewriting. Progress tests progress of the argument strategy and fails if no progress was made, while `try` always succeeds, catching failures. Choice is left-biased: it will launch the first strategy and fall back on the second one in case of failure. One can iterate a strategy at least 1 time using `repeat` and at least 0 times using `any`.

The `subterm` and `subterms` strategies apply their argument strategy `s` to respectively one or all subterms of the current term under consideration, left-to-right. `subterm` stops at the first subterm for which `s` made progress. The composite strategies `innermost` and `outermost` perform a single innermost or outermost rewrite using their argument strategy. Their counterparts `bottomup` and `topdown` perform as many rewritings as possible, starting from the bottom or the top of the term.

Hint databases created for `autorewrite` can also be used by `rewrite_strat` using the `hints` strategy that applies any of the lemmas at the current subterm. The `terms` strategy takes the lemma names directly as arguments. The `eval` strategy expects a reduction expression (see Performing computations) and succeeds if it reduces the subterm under consideration. The `fold` strategy takes a term `c` and tries to unify it to the current subterm, converting it to `c` on success, it is stronger than the tactic `fold`.

### Usage

`rewrite_strat s [in ident]`

  Rewrite using the strategy s in hypothesis ident or the conclusion.

  Error `Nothing to rewrite.`

    If the strategy failed.

  Error `No progress made.`

    If the strategy succeeded but made no progress.

  Error `Unable to satisfy the rewriting constraints.`

    If the strategy succeeded and made progress but the corresponding rewriting constraints are not satisfied.

  The `setoid_rewrite c` tactic is basically equivalent to `rewrite_strat (outermost c)`.