proposition: Lemma subset_set1 A : A \subset [set x] -> (A = set0) \/ (A = [set x]).
proof: Proof. by rewrite -powersetE powerset1 !inE=> /orP[]/eqP->; [left|right]. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(div ssralg ssrint ssrnum binomial) tools combclass sorted ordtype partition