proposition: Lemma equivf_sym : symmetric equivf.
proof: Proof. by move=> x y; rewrite /equivf eq_sym; congr (_==_); rewrite mulrC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(ssrAC choice tuple bigop ssralg poly polydiv) mathcomp(generic_quotient)