Fixpoint interp_PElist (l:list R) (lpe:list (PExpr C * PExpr C)) {struct lpe} : Prop :=
match lpe with
| nil => True
| (me,pe)::lpe =>
match lpe with
| nil => PEeval l me == PEeval l pe
| _ => PEeval l me == PEeval l pe /\ interp_PElist l lpe
end
end.

Lemma ring_correct : forall l pe1 pe2,
(norm_subst pe1 =? norm_subst pe2) = true ->
PEeval l pe1 == PEeval l pe2.