(*memories that do not contain "dangling pointers"*)
Definition mem_wd m := Mem.inject_neutral (Mem.nextblock m) m.

Lemma  mem_wd_drop: forall m b lo hi p m' (DROP: Mem.drop_perm m b lo hi p = Some m')
(WDm: mem_wd m), Mem.valid_block m b -> mem_wd m'.