proposition: Lemma mset_cons a s : seq_mset (a :: s) = a +` (seq_mset s).
proof: Proof. by apply/msetP=> x; rewrite !msetE /= eq_sym. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap