proposition: Lemma msetU1K a B : a \notin B -> (a |` B) `\ a = B.
proof: Proof. move=> aB; apply/msetP=> x; rewrite !msetE. have [->|] := altP eqP; first by rewrite (mset_eq0P _). by rewrite max0n subn0. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap