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 source_SomeI: forall j m b2 b1 ofs1 delta (OV:Mem.meminj_no_overlap j m), j b1 = Some(b2,delta) -> Mem.perm m b1 ofs1 Max Nonempty -> source j m b2 (ofs1+delta) = Some(b1,ofs1).