ewrite -{1 3}(subnKC le_n_m) eqn_add2l; case: eqP => [->|]; rewrite ?mul0rn.
Qed.ewrite !(coef_nderivn, coefD, coefZ) mulrnDl mulrnAr.
Qed.Canonical nderivn_additive n := Additive(nderivn_is_linear