Definition pWF_pplusl : forall pl,
  andl pWF pl -> pWF (pplusl pl).

(* Specifications: *)
Lemma pplusl_correct : forall pl l, peval (pplusl pl) l = plusl (map (fun p => peval p l) pl).
Lemma peval_nth_pplus : forall pl l i n, peval (nth i pl (pcst n 0)) l <= peval (pplusl pl) l.