proposition: Lemma msetUP x A B : reflect (x \in A \/ x \in B) (x \in A `|` B).
proof: Proof. by rewrite !inE; exact: orP. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap