proposition: Lemma mem_mset1U a A : a \in A -> a |` A = A.
proof: Proof. rewrite in_mset => aA; apply/msetP => x; rewrite !msetE (maxn_idPr _) //. by have [->|//] := altP eqP; rewrite (leq_trans _ aA). Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap