proposition: Lemma mderivmDm m1 m2 p : p^`M[m1 + m2] = p^`M[m1]^`M[m2].
proof: Proof. rewrite !mderivm_foldr -foldr_cat; apply/mderiv_perm. apply/seq.permP => /= a; rewrite count_cat !count_flatten. rewrite !sumnE !big_map -big_split /=; apply/eq_bigr=> i _. by rewrite mnmDE nseqD count_cat addnC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(order fingroup perm ssralg zmodp poly ssrint) mathcomp(matrix vector) mathcomp(bigenough) ssrcomplements freeg