proposition: Lemma signr_odd n : (-1) ^+ (odd n) = (-1) ^+ n :> R.
proof: Proof. elim: n => //= n IHn; rewrite exprS -{}IHn. by case/odd: n; rewrite !mulN1r ?opprK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)