Definition polyC c : {poly R} := insubd poly_nil [:: c].
Local Notation "c %:P" := (polyC c).

Lemma size1_polyC p : size p <= 1 -> p = (p`_0)%:P.
Definition cons_poly c p : {poly R} :=
if p is Polynomial ((_ :: _) as s) ns then
@Polynomial R (c :: s) ns
else c%:P.