proposition: Lemma size_derivn (R : realDomainType) (p : {poly R}) n : size p^`(n) = (size p - n)%N.
proof: Proof. elim: n=> [|n ihn]; first by rewrite derivn0 subn0. by rewrite derivnS size_deriv ihn -subnS. Qed.
imports: mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq choice fintype) mathcomp(bigop order ssralg ssrnum ssrint rat poly polydiv polyorder) mathcomp(perm matrix mxpoly polyXY binomial bigenough)