Section CardFunImage.
Variables (T T' : finType) (f : T -> T').
Implicit Type A : {pred T}.
Hypothesis card_range : #|T| >= #|T'|.
Let eq_card : #|T| = #|T'|. Proof. by apply/eqP; rewrite eqn_leq leq_card. Qed.

Lemma inj_card_onto y : y \in codom f.