proposition: Lemma seq_mset_id A : seq_mset A = A.
proof: Proof. by apply/msetP=> a; rewrite mset_seqE count_mem_mset. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap