Goal forall m b ofs p (H: ~ Mem.perm m b ofs p Nonempty), PMap.get b (Mem.mem_access m) ofs p = None.Definition isSource j m1 (b2:block) (ofs2:Z) b : option Z :=
  match j b with None => None
         | Some (bb,delta) => if eq_block b2 bb
                                             then if Mem.perm_dec m1 b (ofs2 - delta) Max Nonempty
                                                     then Some (ofs2 - delta)  else None
                                             else None
  end.

Lemma isSource_NoneE: forall j m b2 ofs2 b1, None = isSource j m b2 ofs2 b1 -> j b1 = None \/ (exists b, exists delta, j b1 = Some (b,delta) /\ b<>b2) \/ (exists delta, j b1 = Some(b2,delta) /\ ~ Mem.perm m b1 (ofs2-delta) Max Nonempty).