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 memvalUnchOnR: forall m1 m2 m3 m3' b ofs
(n : ~ Mem.perm m1 b ofs Max Nonempty)
(UnchOn13 : Mem.unchanged_on (loc_out_of_bounds m1) m3 m3')
(MV : memval_inject inject_id
(ZMap.get ofs (PMap.get b (Mem.mem_contents m2)))
(ZMap.get ofs (PMap.get b (Mem.mem_contents m3))))
(Rd: Mem.perm m3 b ofs Cur Readable),
memval_inject inject_id (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)))
(ZMap.get ofs (PMap.get b (Mem.mem_contents m3'))).