proposition: Lemma subset1 A x : (A \subset [set x]) = (A == [set x]) || (A == set0).
proof: Proof. rewrite eqEcard cards1 -cards_eq0 orbC andbC. by case: posnP => // A0; rewrite (cards0_eq A0) sub0set. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)