(** [or A B], written [A \/ B], is the disjunction of [A] and [B] *)
Inductive or (A B:Prop) : Prop :=
| or_introl : A -> A \/ B
| or_intror : B -> A \/ B
where "A \/ B" := (or A B) : type_scope.
Arguments or_introl [A B] _, [A] B _.
Arguments or_intror [A B] _, A [B] _.
Register or as core.or.type.
(** [iff A B], written [A <-> B], expresses the equivalence of [A] and [B] *)
Definition iff (A B:Prop) := (A -> B) /\ (B -> A).
Notation "A <-> B" := (iff A B) : type_scope.
Register iff as core.iff.type.
Register proj1 as core.iff.proj1.
Register proj2 as core.iff.proj2.
Section Equivalence.

Theorem iff_trans : forall A B C:Prop, (A <-> B) -> (B <-> C) -> (A <-> C).