Fixpoint cur_perm (l: block * Z) (T: list mem_event): option permission :=
  match T with
      nil => None
    | (mu :: R) =>
          let popt := cur_perm l R in
          match mu, l with
            | (Read b ofs n bytes), (b',ofs') =>
                 pmax (if eq_block b b' && zle ofs ofs' && zlt ofs' (ofs+n)
                       then Some Readable else None) popt
            | (Write b ofs bytes), (b',ofs') =>
                 pmax (if eq_block b b' && zle ofs ofs' && zlt ofs' (ofs+ Zlength bytes)
                       then Some Writable else None) popt
            | (Alloc b lo hi), (b',ofs') =>  (*we don't add a constraint relating lo/hi/ofs*)
                 if eq_block b b' then None else popt
            | (Free l), (b',ofs') =>
                 List.fold_right (fun tr qopt => match tr with (b,lo,hi) =>
                                                   if eq_block b b' && zle lo ofs' && zlt ofs' hi
                                                   then Some Freeable else qopt
                                                end)
                                 popt l
          end
  end.

Lemma po_None popt: Mem.perm_order'' popt None.