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