Ltac destr_mfactor R S := match goal with
  | H : context [MFactor ?P _] |- context [MFactor ?P ?M] =>
    specialize (H M); destruct MFactor as (R,S)
 end.

Lemma Mphi_ok P M l : let (Q,R) := MFactor P M in P@l == Q@l + M@@l * R@l.