Definition same_except_cur (m m' : Mem.mem) :=
  Mem.mem_contents m = Mem.mem_contents m' /\
  max_access_at m = max_access_at m' /\
  Mem.nextblock m = Mem.nextblock m'.

Lemma same_perm_spec m1 m2 : Mem.perm m1 = Mem.perm m2 <-> (forall k x, access_at m1 x k = access_at m2 x k).