Section Image.
Variables U V : Type.
Inductive Im (X:Ensemble U) (f:U -> V) : Ensemble V :=
Im_intro : forall x:U, In _ X x -> forall y:V, y = f x -> In _ (Im X f) y.
Definition injective (f:U -> V) := forall x y:U, f x = f y -> x = y.

Lemma not_injective_elim :
forall f:U -> V,
~ injective f ->  exists x : _, (exists y : _, f x = f y /\ x <> y).