Library Coq.Logic.Eqdep_dec
We prove that there is only one proof of x=x, i.e eq_refl x.
    This holds if the equality upon the set of x is decidable.
    A corollary of this theorem is the equality of the right projections
    of two equal dependent pairs.
 
    Author:   Thomas Kleymann |<tms@dcs.ed.ac.uk>| in Lego
              adapted to Coq by B. Barras
 
    Credit:   Proofs up to K_dec follow an outline by Michael Hedberg
 
Table of contents:
 
1. Streicher's K and injectivity of dependent pair hold on decidable types
 
1.1. Definition of the functor that builds properties of dependent equalities
     from a proof of decidability of equality for a set in Type
 
1.2. Definition of the functor that builds properties of dependent equalities
     from a proof of decidability of equality for a set in Set
 
 
Streicher's K and injectivity of dependent pair hold on decidable types
Set Implicit Arguments.
Section EqdepDec.
Variable A : Type.
Let comp (x y y':A) (eq1:x = y) (eq2:x = y') : y = y' :=
eq_ind _ (fun a => a = y') eq2 _ eq1.
Remark trans_sym_eq : forall (x y:A) (u:x = y), comp u u = eq_refl y.
Variable x : A.
Variable eq_dec : forall y:A, x = y \/ x <> y.
Let nu (y:A) (u:x = y) : x = y :=
match eq_dec y with
| or_introl eqxy => eqxy
| or_intror neqxy => False_ind _ (neqxy u)
end.
Let nu_constant : forall (y:A) (u v:x = y), nu u = nu v.
Qed.
Let nu_inv (y:A) (v:x = y) : x = y := comp (nu (eq_refl x)) v.
Remark nu_left_inv_on : forall (y:A) (u:x = y), nu_inv (nu u) = u.
Theorem eq_proofs_unicity_on : forall (y:A) (p1 p2:x = y), p1 = p2.
Theorem K_dec_on :
forall P:x = x -> Prop, P (eq_refl x) -> forall p:x = x, P p.
The corollary 
Let proj (P:A -> Prop) (exP:ex P) (def:P x) : P x :=
match exP with
| ex_intro _ x' prf =>
match eq_dec x' with
| or_introl eqprf => eq_ind x' P prf x (eq_sym eqprf)
| _ => def
end
end.
Theorem inj_right_pair_on :
forall (P:A -> Prop) (y y':P x),
ex_intro P x y = ex_intro P x y' -> y = y'.
End EqdepDec.
Now we prove the versions that require decidable equality for the entire type
    rather than just on the given element.  The rest of the file uses this total
    decidable equality.  We could do everything using decidable equality at a point
    (because the induction rule for eq is really an induction rule for
    { y : A | x = y }), but we don't currently, because changing everything
    would break backward compatibility and no-one has yet taken the time to define
    the pointed versions, and then re-define the non-pointed versions in terms of
    those. 
Theorem eq_proofs_unicity A (eq_dec : forall x y : A, x = y \/ x <> y) (x : A)
: forall (y:A) (p1 p2:x = y), p1 = p2.
Theorem K_dec A (eq_dec : forall x y : A, x = y \/ x <> y) (x : A)
: forall P:x = x -> Prop, P (eq_refl x) -> forall p:x = x, P p.
Theorem inj_right_pair A (eq_dec : forall x y : A, x = y \/ x <> y) (x : A)
: forall (P:A -> Prop) (y y':P x),
ex_intro P x y = ex_intro P x y' -> y = y'.
Require Import EqdepFacts.
We deduce axiom K for (decidable) types 
Theorem K_dec_type :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Theorem K_dec_set :
forall A:Set,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
Theorem K_dec_set :
forall A:Set,
(forall x y:A, {x = y} + {x <> y}) ->
forall (x:A) (P:x = x -> Prop), P (eq_refl x) -> forall p:x = x, P p.
We deduce the eq_rect_eq axiom for (decidable) types 
Theorem eq_rect_eq_dec :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (p:A) (Q:A -> Type) (x:Q p) (h:p = p), x = eq_rect p Q x p h.
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (p:A) (Q:A -> Type) (x:Q p) (h:p = p), x = eq_rect p Q x p h.
We deduce the injectivity of dependent equality for decidable types 
Theorem eq_dep_eq_dec :
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (P:A->Type) (p:A) (x y:P p), eq_dep A P p x p y -> x = y.
Theorem UIP_dec :
forall (A:Type),
(forall x y:A, {x = y} + {x <> y}) ->
forall (x y:A) (p1 p2:x = y), p1 = p2.
Unset Implicit Arguments.
forall A:Type,
(forall x y:A, {x = y} + {x <> y}) ->
forall (P:A->Type) (p:A) (x y:P p), eq_dep A P p x p y -> x = y.
Theorem UIP_dec :
forall (A:Type),
(forall x y:A, {x = y} + {x <> y}) ->
forall (x y:A) (p1 p2:x = y), p1 = p2.
Unset Implicit Arguments.
Definition of the functor that builds properties of dependent equalities on decidable sets in Type
The module DecidableEqDep collects equality properties for decidable
    set in Type 
Invariance by Substitution of Reflexive Equality Proofs 
Injectivity of Dependent Equality 
Uniqueness of Identity Proofs (UIP) 
Uniqueness of Reflexive Identity Proofs 
Streicher's axiom K 
Injectivity of equality on dependent pairs in Type 
Proof-irrelevance on subsets of decidable sets 
Lemma inj_pairP2 :
forall (P:U -> Prop) (x:U) (p q:P x),
ex_intro P x p = ex_intro P x q -> p = q.
End DecidableEqDep.
Definition of the functor that builds properties of dependent equalities on decidable sets in Set
Module Type DecidableSet.
Parameter U:Set.
Axiom eq_dec : forall x y:U, {x = y} + {x <> y}.
End DecidableSet.
The module DecidableEqDepSet collects equality properties for decidable
    set in Set 
Invariance by Substitution of Reflexive Equality Proofs 
Injectivity of Dependent Equality 
Uniqueness of Identity Proofs (UIP) 
Uniqueness of Reflexive Identity Proofs 
Streicher's axiom K 
Proof-irrelevance on subsets of decidable sets 
Injectivity of equality on dependent pairs in Type 
Injectivity of equality on dependent pairs with second component
      in Type 
From decidability to inj_pair2 
Lemma inj_pair2_eq_dec : forall A:Type, (forall x y:A, {x=y}+{x<>y}) ->
( forall (P:A -> Type) (p:A) (x y:P p), existT P p x = existT P p y -> x = y ).
( forall (P:A -> Type) (p:A) (x y:P p), existT P p x = existT P p y -> x = y ).
Examples of short direct proofs of unicity of reflexivity proofs
      on specific domains 
    
  