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

Lemma PSubstL_ok n LM1 P1 P2 l : PSubstL P1 LM1 n = Some P2 -> MPcond LM1 l -> P1@l == P2@l.