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_refl_1: forall p1 p1' (HP:forall p, perm_order p1' p -> perm_order p1 p), perm_order p1 p1'.