proposition: Lemma msetnP n x a : reflect (0 < n /\ x = a) (x \in msetn n a).
proof: Proof. by do [apply: (iffP idP); rewrite !inE] => [/andP[]|[]] -> /eqP. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap