proposition: Lemma msetUE A B C : msetU A B = A `+` (B `\` A).
proof: Proof. by apply/msetP=> a; rewrite !msetE maxnE. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap