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