proposition: Lemma mulrnBr x m n : n <= m -> x *+ (m - n) = x *+ m - x *+ n.
proof: Proof. elim: m n => [|m IHm] [|n le_n_m]; rewrite ?subr0 // {}IHm //. by rewrite mulrSr mulrS opprD addrA addrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)