Section ExtensionalEquality.
Variables A B C : Type.
Definition eqfun (f g : B -> A) : Prop := forall x, f x = g x.
Definition eqrel (r s : C -> B -> A) : Prop := forall x y, r x y = s x y.

Lemma bij_can_eq f' f'' : cancel f f' -> cancel f f'' -> f' =1 f''.