Section CardFunImage.
Variables aT aT2 rT : finType.
Variables (f : aT -> rT) (g : rT -> aT) (f2 : aT -> aT2 -> rT).
Variables (D : {pred aT}) (D2 : {pred aT}).

Lemma card_powerset (T : finType) (A : {set T}) : #|powerset A| = 2 ^ #|A|.