eapply mem_forward_preserve; trivial.
Qed.

Theorem mem_step_max_eq_or_free m m' (STEP: mem_step m m') b (VB: Mem.valid_block m b) ofs:
(forall k p, Mem.perm m b ofs k p <-> Mem.perm m' b ofs k p) \/
(Mem.perm m b ofs Max Freeable /\ None = ((Mem.mem_access m') !! b ofs Max)).