Parameter REACH : mem -> (block -> bool) -> block -> bool.

Lemma REACH_cons: forall m B b b' z off n q,
REACH m B b' = true ->
Mem.perm m b' z Cur Readable ->
ZMap.get z (PMap.get b' (Mem.mem_contents m)) =
Fragment (Vptr b off) q  n ->
REACH m B b = true.