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).