proposition: Lemma mset2P x a b : reflect (x = a \/ x = b) (x \in [mset a; b]).
proof: Proof. by rewrite !inE; apply: (iffP orP) => [] [] /eqP; intuition. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap