proposition: Lemma mulr_signM (b1 b2 : bool) x1 x2 : ((-1) ^+ b1 * x1) * ((-1) ^+ b2 * x2) = (-1) ^+ (b1 (+) b2) * (x1 * x2).
proof: Proof. by rewrite signr_addb -!mulrA; congr (_ * _); rewrite !mulrA commr_sign. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)