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_order_antisym: forall p1 p2, perm_order p1 p2 -> perm_order p2 p1 -> p1=p2.