(* equality of normal forms rather than syntactic equality *)
Fixpoint Fcons0 (e:PExpr C) (l:list (PExpr C)) {struct l} : list (PExpr C) :=
match l with
nil       => cons e nil
| cons a l1 =>
if Peq ceqb (Nnorm O nil e) (Nnorm O nil a) then l
else cons a (Fcons0 e l1)
end.

(* Specifications: *)
Theorem PFcons0_fcons_inv:
forall l a l1, PCond l (Fcons0 a l1) ->  ~ a @ l == 0 /\ PCond l l1.