proposition: Lemma fsetP {A B} : A =i B <-> A = B.
proof: Proof. by split=> [eqAB|-> //]; apply/val_inj/canonical_eq_keys => //= a. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)