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_subst: forall b1 b2 m1 m2 ofs1 ofs2 k (H: PMap.get b1 (Mem.mem_access m1) ofs1 k = PMap.get b2 (Mem.mem_access m2) ofs2 k), Mem.perm m1 b1 ofs1 k = Mem.perm m2 b2 ofs2 k.