proposition: Lemma mulrnDr x m n : x *+ (m + n) = x *+ m + x *+ n.
proof: Proof. elim: m => [|m IHm]; first by rewrite add0r. by rewrite !mulrS IHm addrA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)