proposition: Lemma card_fset (T : choiceType) (A : {fset T}) : #|` [fset x in A] | = #|` A|.
proof: Proof. by rewrite card_imfset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)