Definition  source j m (b2:block) (ofs2:Z) : option(block * Z) :=
if plt 1%positive (Mem.nextblock m)
then sourceN (Pos.pred (Mem.nextblock m)) j m b2 ofs2
else None.

Lemma perm_splitA: forall m b ofs k' p m2' m2 m3' k b3 ofs3
(SPLIT: (Mem.perm m b ofs k' p-> PMap.get b (Mem.mem_access m2') ofs k = PMap.get b (Mem.mem_access m2) ofs k) /\
(~Mem.perm m b ofs k' p -> PMap.get b (Mem.mem_access m2') ofs k = PMap.get b3 (Mem.mem_access m3') ofs3 k))
(P:Prop)
(HA: Mem.perm m b ofs k' p -> Mem.perm m2' b ofs k = Mem.perm m2 b ofs k -> P)
(HB: ~Mem.perm m b ofs k' p -> Mem.perm m2' b ofs k = Mem.perm m3' b3 ofs3 k  -> P), P.