proposition: Lemma size1_polyC p : size p <= 1 -> p = (p`_0)%:P.
proof: Proof. move=> le_p_1; apply/polyP=> i; rewrite coefC. by case: i => // i; rewrite nth_default // (leq_trans le_p_1). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)