proposition: Lemma card_fsub B A : A `<=` B -> #|fsub B A| = #|` A|.
proof: Proof. by move=> sAB; rewrite cardfE fsubE card_imset //; apply: fincl_inj. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)