proposition: Lemma enum_msetE a A : (a \in A) = (a \in flatten [seq nseq (A x) x | x <- finsupp A]).
proof: Proof. by transitivity (a \in enum_mset A); rewrite // unlock. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap