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 approx_eq_app_pred {P1 P2 : mpred} x n : level x < n -> @approx n P1 = approx n P2 -> app_pred P1 x -> app_pred P2 x.