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 source_NoneE: forall j m b2 ofs2, None = source j m b2 ofs2 -> forall b1 delta, (b1 < Mem.nextblock m)%positive -> j b1 = Some(b2,delta) -> ~ Mem.perm m b1 (ofs2-delta) Max Nonempty.