(* Peut-on factoriser ici *)
Infix "--" := NPEsub (at level 50, left associativity).

Theorem NPEsub_ok e1 e2: (e1 -- e2 === e1 - e2)%poly.
Definition NPEopp e1 :=
match e1 with PEc c1 => PEc (- c1) | _ => - e1 end%poly.