proposition: Lemma eqEfcard A B : (A == B) = (A `<=` B) && (#|` B| <= #|` A|)%N.
proof: Proof. rewrite -(inj_in_eq (@fsub_inj (A `|` B))) -?topredE //=. by rewrite eqEcard !(@subset_fsubE (A `|` B)) ?(@card_fsub (A `|` B)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)