proposition: Lemma size_mulX p : p != 0 -> size (p * 'X) = (size p).+1.
proof: Proof. by move/polyseqMX->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)