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_nextblock_gt_1: forall m b ofs k p (P: Mem.perm m b ofs k p), Plt 1 (Mem.nextblock m).