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'))).