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).