proposition: Lemma canonical_eq_keys ks ks' : canonical_keys ks -> canonical_keys ks' -> ks =i ks' -> ks = ks'.
proof: Proof. move=> /eqP; case: _ /; move=> /eqP; case: _ / => eq_ks_ks'. by apply/eq_sort_keys => x; rewrite -sort_keysE eq_ks_ks' sort_keysE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)