proposition: Lemma sort_keys_id ks : sort_keys (sort_keys ks) = sort_keys ks.
proof: Proof. by have /eq_sort_keys := sort_keysE ks. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)