Definition mem_forward (m1 m2:mem) :=
  forall b, Mem.valid_block m1 b ->
    (Mem.valid_block m2 b
     /\ (forall ofs p, Mem.perm m2 b ofs Max p -> Mem.perm m1 b ofs Max p)
     (*/\ readonly m1 b m2*)).

Lemma matchOptE: forall {A} (a:option A) (P: A -> Prop), match a with Some b => P b | None => False end -> exists b, a = Some b /\ P b.