Definition injective (f:U -> V) := forall x y:U, f x = f y -> x = y.

Lemma Pigeonhole_principle :
forall (A:Ensemble U) (f:U -> V) (n:nat),
cardinal _ A n ->
forall n':nat,
cardinal _ (Im A f) n' ->
n' < n ->  exists x : _, (exists y : _, f x = f y /\ x <> y).