Section equality.
Variables A B : Type.
Variable f : A -> B.
Variables x y z : A.
End equality.
Definition eq_sind_r :
forall (A:Type) (x:A) (P:A -> SProp), P x -> forall y:A, y = x -> P y.
Register f_equal2 as core.eq.congr2.
(* Aliases *)
Notation sym_eq := eq_sym (only parsing).
Notation trans_eq := eq_trans (only parsing).
Notation sym_not_eq := not_eq_sym (only parsing).
Notation refl_equal := eq_refl (only parsing).
Notation sym_equal := eq_sym (only parsing).
Notation trans_equal := eq_trans (only parsing).
Notation sym_not_equal := not_eq_sym (only parsing).
Hint Immediate eq_sym not_eq_sym: core.
(** Basic definitions about relations and properties *)
Definition subrelation (A B : Type) (R R' : A->B->Prop) :=
forall x y, R x y -> R' x y.
Definition unique (A : Type) (P : A->Prop) (x:A) :=
P x /\ forall (x':A), P x' -> x=x'.
Definition uniqueness (A:Type) (P:A->Prop) := forall x y, P x -> P y -> x = y.
(** Unique existence *)
Notation "'exists' ! x .. y , p" :=
(ex (unique (fun x => .. (ex (unique (fun y => p))) ..)))
(at level 200, x binder, right associativity,
format "'[' 'exists'  !  '/  ' x  ..  y ,  '/  ' p ']'")
: type_scope.
(** * Being inhabited *)
(** The predicate [inhabited] can be used in different contexts. If [A] is thought as a type, [inhabited A] states that [A] is inhabited. If [A] is thought as a computationally relevant proposition, then [inhabited A] weakens [A] so as to hide its computational meaning. The so-weakened proof remains computationally relevant but only in a propositional context. *)
Inductive inhabited (A:Type) : Prop := inhabits : A -> inhabited A.
Hint Resolve inhabits: core.

Lemma exists_inhabited : forall (A:Type) (P:A->Prop),
(exists x, P x) -> inhabited A.