(** ** Sum of polynomials  - [pplus p1 p2] is the sum of the polynomials [p1] and [p2].  - [pplusl pl] is the sum of the polynomials in the list [pl]. *)
Definition pplus (p1 p2:pol) : pol :=
(max (fst p1) (fst p2), snd p1 ++ snd p2).

Lemma pplus_correct : forall p1 p2 l,
peval (pplus p1 p2) l = peval p1 l + peval p2 l.