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 POneSubst_ok P1 M1 P2 P3 l :
POneSubst P1 M1 P2 = Some P3 -> M1@@l == P2@l ->
P1@l == P3@l.