Fixpoint MPcond (LM1: list (C * Mon * Pol)) (l: list R) : Prop :=
   match LM1 with
   | (M1,P2) :: LM2 => ([fst M1] * (snd M1)@@l == P2@l) /\ MPcond LM2 l
   | _ => True
   end.

Lemma PSubstL1_ok n LM1 P1 l : MPcond LM1 l -> P1@l == (PSubstL1 P1 LM1 n)@l.