proposition: Lemma msetB11 b A : (b \in A `\ b) = (A b > 1).
proof: Proof. by rewrite inE eqxx /= in_mset -geq_max. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap