path: root/theories/Init/Specif.v

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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
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(** Basic specifications : sets that may contain logical information *)

Set Implicit Arguments.
Set Reversible Pattern Implicit.

Require Import Notations.
Require Import Datatypes.
Require Import Logic.

(** Subsets and Sigma-types *)

(** [(sig A P)], or more suggestively [{x:A | P x}], denotes the subset
    of elements of the type [A] which satisfy the predicate [P].
    Similarly [(sig2 A P Q)], or [{x:A | P x & Q x}], denotes the subset
    of elements of the type [A] which satisfy both [P] and [Q]. *)

Inductive sig (A:Type) (P:A -> Prop) : Type :=
    exist : forall x:A, P x -> sig P.

Inductive sig2 (A:Type) (P Q:A -> Prop) : Type :=
    exist2 : forall x:A, P x -> Q x -> sig2 P Q.

(** [(sigT A P)], or more suggestively [{x:A & (P x)}] is a Sigma-type.
    Similarly for [(sigT2 A P Q)], also written [{x:A & (P x) & (Q x)}]. *)

Inductive sigT (A:Type) (P:A -> Type) : Type :=
    existT : forall x:A, P x -> sigT P.

Inductive sigT2 (A:Type) (P Q:A -> Type) : Type :=
    existT2 : forall x:A, P x -> Q x -> sigT2 P Q.

(* Notations *)

Arguments sig (A P)%type.
Arguments sig2 (A P Q)%type.
Arguments sigT (A P)%type.
Arguments sigT2 (A P Q)%type.

Notation "{ x  |  P }" := (sig (fun x => P)) : type_scope.
Notation "{ x  |  P  & Q }" := (sig2 (fun x => P) (fun x => Q)) : type_scope.
Notation "{ x : A  |  P }" := (sig (A:=A) (fun x => P)) : type_scope.
Notation "{ x : A  |  P  & Q }" := (sig2 (A:=A) (fun x => P) (fun x => Q)) :
  type_scope.
Notation "{ x  &  P }" := (sigT (fun x => P)) : type_scope.
Notation "{ x : A  & P }" := (sigT (A:=A) (fun x => P)) : type_scope.
Notation "{ x : A  & P  & Q }" := (sigT2 (A:=A) (fun x => P) (fun x => Q)) :
  type_scope.

Notation "{ ' pat  |  P }" := (sig (fun pat => P)) : type_scope.
Notation "{ ' pat  |  P  & Q }" := (sig2 (fun pat => P) (fun pat => Q)) : type_scope.
Notation "{ ' pat : A  |  P }" := (sig (A:=A) (fun pat => P)) : type_scope.
Notation "{ ' pat : A  |  P  & Q }" := (sig2 (A:=A) (fun pat => P) (fun pat => Q)) :
  type_scope.
Notation "{ ' pat : A  & P }" := (sigT (A:=A) (fun pat => P)) : type_scope.
Notation "{ ' pat : A  & P  & Q }" := (sigT2 (A:=A) (fun pat => P) (fun pat => Q)) :
  type_scope.

Add Printing Let sig.
Add Printing Let sig2.
Add Printing Let sigT.
Add Printing Let sigT2.


(** Projections of [sig]

    An element [y] of a subset [{x:A | (P x)}] is the pair of an [a]
    of type [A] and of a proof [h] that [a] satisfies [P].  Then
    [(proj1_sig y)] is the witness [a] and [(proj2_sig y)] is the
    proof of [(P a)] *)

(* Set Universe Polymorphism. *)
Section Subset_projections.

  Variable A : Type.
  Variable P : A -> Prop.

  Definition proj1_sig (e:sig P) := match e with
                                    | exist _ a b => a
                                    end.

  Definition proj2_sig (e:sig P) :=
    match e return P (proj1_sig e) with
    | exist _ a b => b
    end.

End Subset_projections.


(** [sig2] of a predicate can be projected to a [sig].

    This allows [proj1_sig] and [proj2_sig] to be usable with [sig2].

    The [let] statements occur in the body of the [exist] so that
    [proj1_sig] of a coerced [X : sig2 P Q] will unify with [let (a,
    _, _) := X in a] *)

Definition sig_of_sig2 (A : Type) (P Q : A -> Prop) (X : sig2 P Q) : sig P
  := exist P
           (let (a, _, _) := X in a)
           (let (x, p, _) as s return (P (let (a, _, _) := s in a)) := X in p).

(** Projections of [sig2]

    An element [y] of a subset [{x:A | (P x) & (Q x)}] is the triple
    of an [a] of type [A], a of a proof [h] that [a] satisfies [P],
    and a proof [h'] that [a] satisfies [Q].  Then
    [(proj1_sig (sig_of_sig2 y))] is the witness [a],
    [(proj2_sig (sig_of_sig2 y))] is the proof of [(P a)], and
    [(proj3_sig y)] is the proof of [(Q a)]. *)

Section Subset_projections2.

  Variable A : Type.
  Variables P Q : A -> Prop.

  Definition proj3_sig (e : sig2 P Q) :=
    let (a, b, c) return Q (proj1_sig (sig_of_sig2 e)) := e in c.

End Subset_projections2.


(** Projections of [sigT]

    An element [x] of a sigma-type [{y:A & P y}] is a dependent pair
    made of an [a] of type [A] and an [h] of type [P a].  Then,
    [(projT1 x)] is the first projection and [(projT2 x)] is the
    second projection, the type of which depends on the [projT1]. *)



Section Projections.

  Variable A : Type.
  Variable P : A -> Type.

  Definition projT1 (x:sigT P) : A := match x with
                                      | existT _ a _ => a
                                      end.

  Definition projT2 (x:sigT P) : P (projT1 x) :=
    match x return P (projT1 x) with
    | existT _ _ h => h
    end.

End Projections.

(** [sigT2] of a predicate can be projected to a [sigT].

    This allows [projT1] and [projT2] to be usable with [sigT2].

    The [let] statements occur in the body of the [existT] so that
    [projT1] of a coerced [X : sigT2 P Q] will unify with [let (a,
    _, _) := X in a] *)

Definition sigT_of_sigT2 (A : Type) (P Q : A -> Type) (X : sigT2 P Q) : sigT P
  := existT P
            (let (a, _, _) := X in a)
            (let (x, p, _) as s return (P (let (a, _, _) := s in a)) := X in p).

(** Projections of [sigT2]

    An element [x] of a sigma-type [{y:A & P y & Q y}] is a dependent
    pair made of an [a] of type [A], an [h] of type [P a], and an [h']
    of type [Q a].  Then, [(projT1 (sigT_of_sigT2 x))] is the first
    projection, [(projT2 (sigT_of_sigT2 x))] is the second projection,
    and [(projT3 x)] is the third projection, the types of which
    depends on the [projT1]. *)

Section Projections2.

  Variable A : Type.
  Variables P Q : A -> Type.

  Definition projT3 (e : sigT2 P Q) :=
    let (a, b, c) return Q (projT1 (sigT_of_sigT2 e)) := e in c.

End Projections2.

(** [sigT] of a predicate is equivalent to [sig] *)

Definition sig_of_sigT (A : Type) (P : A -> Prop) (X : sigT P) : sig P
  := exist P (projT1 X) (projT2 X).

Definition sigT_of_sig (A : Type) (P : A -> Prop) (X : sig P) : sigT P
  := existT P (proj1_sig X) (proj2_sig X).

(** [sigT2] of a predicate is equivalent to [sig2] *)

Definition sig2_of_sigT2 (A : Type) (P Q : A -> Prop) (X : sigT2 P Q) : sig2 P Q
  := exist2 P Q (projT1 (sigT_of_sigT2 X)) (projT2 (sigT_of_sigT2 X)) (projT3 X).

Definition sigT2_of_sig2 (A : Type) (P Q : A -> Prop) (X : sig2 P Q) : sigT2 P Q
  := existT2 P Q (proj1_sig (sig_of_sig2 X)) (proj2_sig (sig_of_sig2 X)) (proj3_sig X).

(** η Principles *)
Definition sigT_eta {A P} (p : { a : A & P a })
  : p = existT _ (projT1 p) (projT2 p).
Proof. destruct p; reflexivity. Defined.

Definition sig_eta {A P} (p : { a : A | P a })
  : p = exist _ (proj1_sig p) (proj2_sig p).
Proof. destruct p; reflexivity. Defined.

Definition sigT2_eta {A P Q} (p : { a : A & P a & Q a })
  : p = existT2 _ _ (projT1 (sigT_of_sigT2 p)) (projT2 (sigT_of_sigT2 p)) (projT3 p).
Proof. destruct p; reflexivity. Defined.

Definition sig2_eta {A P Q} (p : { a : A | P a & Q a })
  : p = exist2 _ _ (proj1_sig (sig_of_sig2 p)) (proj2_sig (sig_of_sig2 p)) (proj3_sig p).
Proof. destruct p; reflexivity. Defined.

(** [exists x : A, B] is equivalent to [inhabited {x : A | B}] *)
Lemma exists_to_inhabited_sig {A P} : (exists x : A, P x) -> inhabited {x : A | P x}.
Proof.
  intros [x y]. exact (inhabits (exist _ x y)).
Qed.

Lemma inhabited_sig_to_exists {A P} : inhabited {x : A | P x} -> exists x : A, P x.
Proof.
  intros [[x y]];exists x;exact y.
Qed.

(** Equality of sigma types *)
Import EqNotations.
Local Notation "'rew' 'dependent' H 'in' H'"
  := (match H with
      | eq_refl => H'
      end)
       (at level 10, H' at level 10,
        format "'[' 'rew'  'dependent'  '/    ' H  in  '/' H' ']'").

(** Equality for [sigT] *)
Section sigT.
  Local Unset Implicit Arguments.
  (** Projecting an equality of a pair to equality of the first components *)
  Definition projT1_eq {A} {P : A -> Type} {u v : { a : A & P a }} (p : u = v)
    : projT1 u = projT1 v
    := f_equal (@projT1 _ _) p.

  (** Projecting an equality of a pair to equality of the second components *)
  Definition projT2_eq {A} {P : A -> Type} {u v : { a : A & P a }} (p : u = v)
    : rew projT1_eq p in projT2 u = projT2 v
    := rew dependent p in eq_refl.

  (** Equality of [sigT] is itself a [sigT] (forwards-reasoning version) *)
  Definition eq_existT_uncurried {A : Type} {P : A -> Type} {u1 v1 : A} {u2 : P u1} {v2 : P v1}
             (pq : { p : u1 = v1 & rew p in u2 = v2 })
    : existT _ u1 u2 = existT _ v1 v2.
  Proof.
    destruct pq as [p q].
    destruct q; simpl in *.
    destruct p; reflexivity.
  Defined.

  (** Equality of [sigT] is itself a [sigT] (backwards-reasoning version) *)
  Definition eq_sigT_uncurried {A : Type} {P : A -> Type} (u v : { a : A & P a })
             (pq : { p : projT1 u = projT1 v & rew p in projT2 u = projT2 v })
    : u = v.
  Proof.
    destruct u as [u1 u2], v as [v1 v2]; simpl in *.
    apply eq_existT_uncurried; exact pq.
  Defined.

  (** Curried version of proving equality of sigma types *)
  Definition eq_sigT {A : Type} {P : A -> Type} (u v : { a : A & P a })
             (p : projT1 u = projT1 v) (q : rew p in projT2 u = projT2 v)
    : u = v
    := eq_sigT_uncurried u v (existT _ p q).

  (** Equality of [sigT] when the property is an hProp *)
  Definition eq_sigT_hprop {A P} (P_hprop : forall (x : A) (p q : P x), p = q)
             (u v : { a : A & P a })
             (p : projT1 u = projT1 v)
    : u = v
    := eq_sigT u v p (P_hprop _ _ _).

  (** Equivalence of equality of [sigT] with a [sigT] of equality *)
  (** We could actually prove an isomorphism here, and not just [<->],
      but for simplicity, we don't. *)
  Definition eq_sigT_uncurried_iff {A P}
             (u v : { a : A & P a })
    : u = v <-> { p : projT1 u = projT1 v & rew p in projT2 u = projT2 v }.
  Proof.
    split; [ intro; subst; exists eq_refl; reflexivity | apply eq_sigT_uncurried ].
  Defined.

  (** Induction principle for [@eq (sigT _)] *)
  Definition eq_sigT_rect {A P} {u v : { a : A & P a }} (Q : u = v -> Type)
             (f : forall p q, Q (eq_sigT u v p q))
    : forall p, Q p.
  Proof. intro p; specialize (f (projT1_eq p) (projT2_eq p)); destruct u, p; exact f. Defined.
  Definition eq_sigT_rec {A P u v} (Q : u = v :> { a : A & P a } -> Set) := eq_sigT_rect Q.
  Definition eq_sigT_ind {A P u v} (Q : u = v :> { a : A & P a } -> Prop) := eq_sigT_rec Q.

  (** Equivalence of equality of [sigT] involving hProps with equality of the first components *)
  Definition eq_sigT_hprop_iff {A P} (P_hprop : forall (x : A) (p q : P x), p = q)
             (u v : { a : A & P a })
    : u = v <-> (projT1 u = projT1 v)
    := conj (fun p => f_equal (@projT1 _ _) p) (eq_sigT_hprop P_hprop u v).

  (** Non-dependent classification of equality of [sigT] *)
  Definition eq_sigT_nondep {A B : Type} (u v : { a : A & B })
             (p : projT1 u = projT1 v) (q : projT2 u = projT2 v)
    : u = v
    := @eq_sigT _ _ u v p (eq_trans (rew_const _ _) q).

  (** Classification of transporting across an equality of [sigT]s *)
  Lemma rew_sigT {A x} {P : A -> Type} (Q : forall a, P a -> Prop) (u : { p : P x & Q x p }) {y} (H : x = y)
    : rew [fun a => { p : P a & Q a p }] H in u
      = existT
          (Q y)
          (rew H in projT1 u)
          (rew dependent H in (projT2 u)).
  Proof.
    destruct H, u; reflexivity.
  Defined.
End sigT.

(** Equality for [sig] *)
Section sig.
  Local Unset Implicit Arguments.
  (** Projecting an equality of a pair to equality of the first components *)
  Definition proj1_sig_eq {A} {P : A -> Prop} {u v : { a : A | P a }} (p : u = v)
    : proj1_sig u = proj1_sig v
    := f_equal (@proj1_sig _ _) p.

  (** Projecting an equality of a pair to equality of the second components *)
  Definition proj2_sig_eq {A} {P : A -> Prop} {u v : { a : A | P a }} (p : u = v)
    : rew proj1_sig_eq p in proj2_sig u = proj2_sig v
    := rew dependent p in eq_refl.

  (** Equality of [sig] is itself a [sig] (forwards-reasoning version) *)
  Definition eq_exist_uncurried {A : Type} {P : A -> Prop} {u1 v1 : A} {u2 : P u1} {v2 : P v1}
             (pq : { p : u1 = v1 | rew p in u2 = v2 })
    : exist _ u1 u2 = exist _ v1 v2.
  Proof.
    destruct pq as [p q].
    destruct q; simpl in *.
    destruct p; reflexivity.
  Defined.

  (** Equality of [sig] is itself a [sig] (backwards-reasoning version) *)
  Definition eq_sig_uncurried {A : Type} {P : A -> Prop} (u v : { a : A | P a })
             (pq : { p : proj1_sig u = proj1_sig v | rew p in proj2_sig u = proj2_sig v })
    : u = v.
  Proof.
    destruct u as [u1 u2], v as [v1 v2]; simpl in *.
    apply eq_exist_uncurried; exact pq.
  Defined.

  (** Curried version of proving equality of sigma types *)
  Definition eq_sig {A : Type} {P : A -> Prop} (u v : { a : A | P a })
             (p : proj1_sig u = proj1_sig v) (q : rew p in proj2_sig u = proj2_sig v)
    : u = v
    := eq_sig_uncurried u v (exist _ p q).

  (** Induction principle for [@eq (sig _)] *)
  Definition eq_sig_rect {A P} {u v : { a : A | P a }} (Q : u = v -> Type)
             (f : forall p q, Q (eq_sig u v p q))
    : forall p, Q p.
  Proof. intro p; specialize (f (proj1_sig_eq p) (proj2_sig_eq p)); destruct u, p; exact f. Defined.
  Definition eq_sig_rec {A P u v} (Q : u = v :> { a : A | P a } -> Set) := eq_sig_rect Q.
  Definition eq_sig_ind {A P u v} (Q : u = v :> { a : A | P a } -> Prop) := eq_sig_rec Q.

  (** Equality of [sig] when the property is an hProp *)
  Definition eq_sig_hprop {A} {P : A -> Prop} (P_hprop : forall (x : A) (p q : P x), p = q)
             (u v : { a : A | P a })
             (p : proj1_sig u = proj1_sig v)
    : u = v
    := eq_sig u v p (P_hprop _ _ _).

  (** Equivalence of equality of [sig] with a [sig] of equality *)
  (** We could actually prove an isomorphism here, and not just [<->],
      but for simplicity, we don't. *)
  Definition eq_sig_uncurried_iff {A} {P : A -> Prop}
             (u v : { a : A | P a })
    : u = v <-> { p : proj1_sig u = proj1_sig v | rew p in proj2_sig u = proj2_sig v }.
  Proof.
    split; [ intro; subst; exists eq_refl; reflexivity | apply eq_sig_uncurried ].
  Defined.

  (** Equivalence of equality of [sig] involving hProps with equality of the first components *)
  Definition eq_sig_hprop_iff {A} {P : A -> Prop} (P_hprop : forall (x : A) (p q : P x), p = q)
             (u v : { a : A | P a })
    : u = v <-> (proj1_sig u = proj1_sig v)
    := conj (fun p => f_equal (@proj1_sig _ _) p) (eq_sig_hprop P_hprop u v).

  Lemma rew_sig {A x} {P : A -> Type} (Q : forall a, P a -> Prop) (u : { p : P x | Q x p }) {y} (H : x = y)
    : rew [fun a => { p : P a | Q a p }] H in u
      = exist
          (Q y)
          (rew H in proj1_sig u)
          (rew dependent H in proj2_sig u).
  Proof.
    destruct H, u; reflexivity.
  Defined.
End sig.

(** Equality for [sigT] *)
Section sigT2.
  (* We make [sigT_of_sigT2] a coercion so we can use [projT1], [projT2] on [sigT2] *)
  Local Coercion sigT_of_sigT2 : sigT2 >-> sigT.
  Local Unset Implicit Arguments.
  (** Projecting an equality of a pair to equality of the first components *)
  Definition sigT_of_sigT2_eq {A} {P Q : A -> Type} {u v : { a : A & P a & Q a }} (p : u = v)
    : u = v :> { a : A & P a }
    := f_equal _ p.
  Definition projT1_of_sigT2_eq {A} {P Q : A -> Type} {u v : { a : A & P a & Q a }} (p : u = v)
    : projT1 u = projT1 v
    := projT1_eq (sigT_of_sigT2_eq p).

  (** Projecting an equality of a pair to equality of the second components *)
  Definition projT2_of_sigT2_eq {A} {P Q : A -> Type} {u v : { a : A & P a & Q a }} (p : u = v)
    : rew projT1_of_sigT2_eq p in projT2 u = projT2 v
    := rew dependent p in eq_refl.

  (** Projecting an equality of a pair to equality of the third components *)
  Definition projT3_eq {A} {P Q : A -> Type} {u v : { a : A & P a & Q a }} (p : u = v)
    : rew projT1_of_sigT2_eq p in projT3 u = projT3 v
    := rew dependent p in eq_refl.

  (** Equality of [sigT2] is itself a [sigT2] (forwards-reasoning version) *)
  Definition eq_existT2_uncurried {A : Type} {P Q : A -> Type}
             {u1 v1 : A} {u2 : P u1} {v2 : P v1} {u3 : Q u1} {v3 : Q v1}
             (pqr : { p : u1 = v1
                    & rew p in u2 = v2 & rew p in u3 = v3 })
    : existT2 _ _ u1 u2 u3 = existT2 _ _ v1 v2 v3.
  Proof.
    destruct pqr as [p q r].
    destruct r, q, p; simpl.
    reflexivity.
  Defined.

  (** Equality of [sigT2] is itself a [sigT2] (backwards-reasoning version) *)
  Definition eq_sigT2_uncurried {A : Type} {P Q : A -> Type} (u v : { a : A & P a & Q a })
             (pqr : { p : projT1 u = projT1 v
                    & rew p in projT2 u = projT2 v & rew p in projT3 u = projT3 v })
    : u = v.
  Proof.
    destruct u as [u1 u2 u3], v as [v1 v2 v3]; simpl in *.
    apply eq_existT2_uncurried; exact pqr.
  Defined.

  (** Curried version of proving equality of sigma types *)
  Definition eq_sigT2 {A : Type} {P Q : A -> Type} (u v : { a : A & P a & Q a })
             (p : projT1 u = projT1 v)
             (q : rew p in projT2 u = projT2 v)
             (r : rew p in projT3 u = projT3 v)
    : u = v
    := eq_sigT2_uncurried u v (existT2 _ _ p q r).

  (** Equality of [sigT2] when the second property is an hProp *)
  Definition eq_sigT2_hprop {A P Q} (Q_hprop : forall (x : A) (p q : Q x), p = q)
             (u v : { a : A & P a & Q a })
             (p : u = v :> { a : A & P a })
    : u = v
    := eq_sigT2 u v (projT1_eq p) (projT2_eq p) (Q_hprop _ _ _).

  (** Equivalence of equality of [sigT2] with a [sigT2] of equality *)
  (** We could actually prove an isomorphism here, and not just [<->],
      but for simplicity, we don't. *)
  Definition eq_sigT2_uncurried_iff {A P Q}
             (u v : { a : A & P a & Q a })
    : u = v
      <-> { p : projT1 u = projT1 v
          & rew p in projT2 u = projT2 v & rew p in projT3 u = projT3 v }.
  Proof.
    split; [ intro; subst; exists eq_refl; reflexivity | apply eq_sigT2_uncurried ].
  Defined.

  (** Induction principle for [@eq (sigT2 _ _)] *)
  Definition eq_sigT2_rect {A P Q} {u v : { a : A & P a & Q a }} (R : u = v -> Type)
             (f : forall p q r, R (eq_sigT2 u v p q r))
    : forall p, R p.
  Proof.
    intro p.
    specialize (f (projT1_of_sigT2_eq p) (projT2_of_sigT2_eq p) (projT3_eq p)).
    destruct u, p; exact f.
  Defined.
  Definition eq_sigT2_rec {A P Q u v} (R : u = v :> { a : A & P a & Q a } -> Set) := eq_sigT2_rect R.
  Definition eq_sigT2_ind {A P Q u v} (R : u = v :> { a : A & P a & Q a } -> Prop) := eq_sigT2_rec R.

  (** Equivalence of equality of [sigT2] involving hProps with equality of the first components *)
  Definition eq_sigT2_hprop_iff {A P Q} (Q_hprop : forall (x : A) (p q : Q x), p = q)
             (u v : { a : A & P a & Q a })
    : u = v <-> (u = v :> { a : A & P a })
    := conj (fun p => f_equal (@sigT_of_sigT2 _ _ _) p) (eq_sigT2_hprop Q_hprop u v).

  (** Non-dependent classification of equality of [sigT] *)
  Definition eq_sigT2_nondep {A B C : Type} (u v : { a : A & B & C })
             (p : projT1 u = projT1 v) (q : projT2 u = projT2 v) (r : projT3 u = projT3 v)
    : u = v
    := @eq_sigT2 _ _ _ u v p (eq_trans (rew_const _ _) q) (eq_trans (rew_const _ _) r).

  (** Classification of transporting across an equality of [sigT2]s *)
  Lemma rew_sigT2 {A x} {P : A -> Type} (Q R : forall a, P a -> Prop)
        (u : { p : P x & Q x p & R x p })
        {y} (H : x = y)
    : rew [fun a => { p : P a & Q a p & R a p }] H in u
      = existT2
          (Q y)
          (R y)
          (rew H in projT1 u)
          (rew dependent H in projT2 u)
          (rew dependent H in projT3 u).
  Proof.
    destruct H, u; reflexivity.
  Defined.
End sigT2.

(** Equality for [sig2] *)
Section sig2.
  (* We make [sig_of_sig2] a coercion so we can use [proj1], [proj2] on [sig2] *)
  Local Coercion sig_of_sig2 : sig2 >-> sig.
  Local Unset Implicit Arguments.
  (** Projecting an equality of a pair to equality of the first components *)
  Definition sig_of_sig2_eq {A} {P Q : A -> Prop} {u v : { a : A | P a & Q a }} (p : u = v)
    : u = v :> { a : A | P a }
    := f_equal _ p.
  Definition proj1_sig_of_sig2_eq {A} {P Q : A -> Prop} {u v : { a : A | P a & Q a }} (p : u = v)
    : proj1_sig u = proj1_sig v
    := proj1_sig_eq (sig_of_sig2_eq p).

  (** Projecting an equality of a pair to equality of the second components *)
  Definition proj2_sig_of_sig2_eq {A} {P Q : A -> Prop} {u v : { a : A | P a & Q a }} (p : u = v)
    : rew proj1_sig_of_sig2_eq p in proj2_sig u = proj2_sig v
    := rew dependent p in eq_refl.

  (** Projecting an equality of a pair to equality of the third components *)
  Definition proj3_sig_eq {A} {P Q : A -> Prop} {u v : { a : A | P a & Q a }} (p : u = v)
    : rew proj1_sig_of_sig2_eq p in proj3_sig u = proj3_sig v
    := rew dependent p in eq_refl.

  (** Equality of [sig2] is itself a [sig2] (fowards-reasoning version) *)
  Definition eq_exist2_uncurried {A} {P Q : A -> Prop}
             {u1 v1 : A} {u2 : P u1} {v2 : P v1} {u3 : Q u1} {v3 : Q v1}
             (pqr : { p : u1 = v1
                    | rew p in u2 = v2 & rew p in u3 = v3 })
    : exist2 _ _ u1 u2 u3 = exist2 _ _ v1 v2 v3.
  Proof.
    destruct pqr as [p q r].
    destruct r, q, p; simpl.
    reflexivity.
  Defined.

  (** Equality of [sig2] is itself a [sig2] (backwards-reasoning version) *)
  Definition eq_sig2_uncurried {A} {P Q : A -> Prop} (u v : { a : A | P a & Q a })
             (pqr : { p : proj1_sig u = proj1_sig v
                    | rew p in proj2_sig u = proj2_sig v & rew p in proj3_sig u = proj3_sig v })
    : u = v.
  Proof.
    destruct u as [u1 u2 u3], v as [v1 v2 v3]; simpl in *.
    apply eq_exist2_uncurried; exact pqr.
  Defined.

  (** Curried version of proving equality of sigma types *)
  Definition eq_sig2 {A} {P Q : A -> Prop} (u v : { a : A | P a & Q a })
             (p : proj1_sig u = proj1_sig v)
             (q : rew p in proj2_sig u = proj2_sig v)
             (r : rew p in proj3_sig u = proj3_sig v)
    : u = v
    := eq_sig2_uncurried u v (exist2 _ _ p q r).

  (** Equality of [sig2] when the second property is an hProp *)
  Definition eq_sig2_hprop {A} {P Q : A -> Prop} (Q_hprop : forall (x : A) (p q : Q x), p = q)
             (u v : { a : A | P a & Q a })
             (p : u = v :> { a : A | P a })
    : u = v
    := eq_sig2 u v (proj1_sig_eq p) (proj2_sig_eq p) (Q_hprop _ _ _).

  (** Equivalence of equality of [sig2] with a [sig2] of equality *)
  (** We could actually prove an isomorphism here, and not just [<->],
      but for simplicity, we don't. *)
  Definition eq_sig2_uncurried_iff {A P Q}
             (u v : { a : A | P a & Q a })
    : u = v
      <-> { p : proj1_sig u = proj1_sig v
          | rew p in proj2_sig u = proj2_sig v & rew p in proj3_sig u = proj3_sig v }.
  Proof.
    split; [ intro; subst; exists eq_refl; reflexivity | apply eq_sig2_uncurried ].
  Defined.

  (** Induction principle for [@eq (sig2 _ _)] *)
  Definition eq_sig2_rect {A P Q} {u v : { a : A | P a & Q a }} (R : u = v -> Type)
             (f : forall p q r, R (eq_sig2 u v p q r))
    : forall p, R p.
  Proof.
    intro p.
    specialize (f (proj1_sig_of_sig2_eq p) (proj2_sig_of_sig2_eq p) (proj3_sig_eq p)).
    destruct u, p; exact f.
  Defined.
  Definition eq_sig2_rec {A P Q u v} (R : u = v :> { a : A | P a & Q a } -> Set) := eq_sig2_rect R.
  Definition eq_sig2_ind {A P Q u v} (R : u = v :> { a : A | P a & Q a } -> Prop) := eq_sig2_rec R.

  (** Equivalence of equality of [sig2] involving hProps with equality of the first components *)
  Definition eq_sig2_hprop_iff {A} {P Q : A -> Prop} (Q_hprop : forall (x : A) (p q : Q x), p = q)
             (u v : { a : A | P a & Q a })
    : u = v <-> (u = v :> { a : A | P a })
    := conj (fun p => f_equal (@sig_of_sig2 _ _ _) p) (eq_sig2_hprop Q_hprop u v).

  (** Non-dependent classification of equality of [sig] *)
  Definition eq_sig2_nondep {A} {B C : Prop} (u v : @sig2 A (fun _ => B) (fun _ => C))
             (p : proj1_sig u = proj1_sig v) (q : proj2_sig u = proj2_sig v) (r : proj3_sig u = proj3_sig v)
    : u = v
    := @eq_sig2 _ _ _ u v p (eq_trans (rew_const _ _) q) (eq_trans (rew_const _ _) r).

  (** Classification of transporting across an equality of [sig2]s *)
  Lemma rew_sig2 {A x} {P : A -> Type} (Q R : forall a, P a -> Prop)
        (u : { p : P x | Q x p & R x p })
        {y} (H : x = y)
    : rew [fun a => { p : P a | Q a p & R a p }] H in u
      = exist2
          (Q y)
          (R y)
          (rew H in proj1_sig u)
          (rew dependent H in proj2_sig u)
          (rew dependent H in proj3_sig u).
  Proof.
    destruct H, u; reflexivity.
  Defined.
End sig2.


(** [sumbool] is a boolean type equipped with the justification of
    their value *)

Inductive sumbool (A B:Prop) : Set :=
  | left : A -> {A} + {B}
  | right : B -> {A} + {B}
 where "{ A } + { B }" := (sumbool A B) : type_scope.

Add Printing If sumbool.

Arguments left {A B} _, [A] B _.
Arguments right {A B} _ , A [B] _.

(** [sumor] is an option type equipped with the justification of why
    it may not be a regular value *)

Inductive sumor (A:Type) (B:Prop) : Type :=
  | inleft : A -> A + {B}
  | inright : B -> A + {B}
 where "A + { B }" := (sumor A B) : type_scope.

Add Printing If sumor.

Arguments inleft {A B} _ , [A] B _.
Arguments inright {A B} _ , A [B] _.

(* Unset Universe Polymorphism. *)

(** Various forms of the axiom of choice for specifications *)

Section Choice_lemmas.

  Variables S S' : Set.
  Variable R : S -> S' -> Prop.
  Variable R' : S -> S' -> Set.
  Variables R1 R2 : S -> Prop.

  Lemma Choice :
   (forall x:S, {y:S' | R x y}) -> {f:S -> S' | forall z:S, R z (f z)}.
  Proof.
   intro H.
   exists (fun z => proj1_sig (H z)).
   intro z; destruct (H z); assumption.
  Defined.

  Lemma Choice2 :
   (forall x:S, {y:S' & R' x y}) -> {f:S -> S' & forall z:S, R' z (f z)}.
  Proof.
    intro H.
    exists (fun z => projT1 (H z)).
    intro z; destruct (H z); assumption.
  Defined.

  Lemma bool_choice :
   (forall x:S, {R1 x} + {R2 x}) ->
     {f:S -> bool | forall x:S, f x = true /\ R1 x \/ f x = false /\ R2 x}.
  Proof.
    intro H.
    exists (fun z:S => if H z then true else false).
    intro z; destruct (H z); auto.
  Defined.

End Choice_lemmas.

Section Dependent_choice_lemmas.

  Variables X : Set.
  Variable R : X -> X -> Prop.

  Lemma dependent_choice :
    (forall x:X, {y | R x y}) ->
    forall x0, {f : nat -> X | f O = x0 /\ forall n, R (f n) (f (S n))}.
  Proof.
    intros H x0.
    set (f:=fix f n := match n with O => x0 | S n' => proj1_sig (H (f n')) end).
    exists f.
    split. reflexivity.
    induction n; simpl; apply proj2_sig.
  Defined.

End Dependent_choice_lemmas.


 (** A result of type [(Exc A)] is either a normal value of type [A] or
     an [error] :

     [Inductive Exc [A:Type] : Type := value : A->(Exc A) | error : (Exc A)].

     It is implemented using the option type. *)
Section Exc.
  Variable A : Type.

  Definition Exc := option A.
  Definition value := @Some A.
  Definition error := @None A.
End Exc.
Arguments error {A}.

Definition except := False_rec. (* for compatibility with previous versions *)

Arguments except [P] _.

Theorem absurd_set : forall (A:Prop) (C:Set), A -> ~ A -> C.
Proof.
  intros A C h1 h2.
  apply False_rec.
  apply (h2 h1).
Defined.

Hint Resolve left right inleft inright: core.
Hint Resolve exist exist2 existT existT2: core.

(* Compatibility *)

Notation sigS := sigT (compat "8.7").
Notation existS := existT (compat "8.7").
Notation sigS_rect := sigT_rect (compat "8.7").
Notation sigS_rec := sigT_rec (compat "8.7").
Notation sigS_ind := sigT_ind (compat "8.7").
Notation projS1 := projT1 (compat "8.7").
Notation projS2 := projT2 (compat "8.7").

Notation sigS2 := sigT2 (compat "8.7").
Notation existS2 := existT2 (compat "8.7").
Notation sigS2_rect := sigT2_rect (compat "8.7").
Notation sigS2_rec := sigT2_rec (compat "8.7").
Notation sigS2_ind := sigT2_ind (compat "8.7").