Definition permMapJoin (pmap1 pmap2 pmap3: access_map) :=
forall b ofs,
permjoin ((pmap1 !! b) ofs) ((pmap2 !! b) ofs) ((pmap3 !! b) ofs).

Lemma permMapLt_invalid_block:
forall pmap m b ofs
(Hlt: permMapLt pmap (getMaxPerm m))
(Hinvalid: ~ Mem.valid_block m b),
(pmap !! b ofs) = None.