proposition: Lemma signrMK n : @involutive R ( *%R ((-1) ^+ n)).
proof: Proof. by move=> x; rewrite mulrA -expr2 sqrr_sign mul1r. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)