(** * Equality *)
(** [eq x y], or simply [x=y] expresses the equality of [x] and [y]. Both [x] and [y] must belong to the same type [A]. The definition is inductive and states the reflexivity of the equality. The others properties (symmetry, transitivity, replacement of equals by equals) are proved below. The type of [x] and [y] can be made explicit using the notation [x = y :> A]. This is Leibniz equality as it expresses that [x] and [y] are equal iff every property on [A] which is true of [x] is also true of [y] *)
Inductive eq (A:Type) (x:A) : A -> Prop :=
eq_refl : x = x :>A
where "x = y :> A" := (@eq A x y) : type_scope.
Arguments eq {A} x _.
Arguments eq_refl {A x} , [A] x.
Arguments eq_ind [A] x P _ y _ : rename.
Arguments eq_rec [A] x P _ y _ : rename.
Arguments eq_rect [A] x P _ y _ : rename.
Notation "x = y" := (eq x y) : type_scope.
Notation "x <> y  :> T" := (~ x = y :>T) : type_scope.
Notation "x <> y" := (~ (x = y)) : type_scope.
#[global]
Hint Resolve I conj or_introl or_intror : core.
#[global]
Hint Resolve eq_refl: core.
#[global]
Hint Resolve ex_intro ex_intro2: core.
Register eq as core.eq.type.
Register eq_refl as core.eq.refl.
Register eq_ind as core.eq.ind.
Register eq_rect as core.eq.rect.
Section Logic_lemmas.

(* Specifications: *)
Theorem absurd : forall A C:Prop, A -> ~ A -> C.
Section equality.