anchor.diff:      BackPs0 = [{StreamNum, []}],
       {P2, BackPs1} = write_stream_pages(P1, Pages, PageSize, BackPs0, StreamNum),
       LastLPN = hd(proplists:get_value(StreamNum, BackPs1)),
  -    io:format(user, "\nLastLPN ~p\n", [LastLPN]),
   
       LastLPN=LastLPN,
       [begin
  
positive.code:          %% oldest -> newest (i.e. smallest LPN to largest LPN) order
           %% together with the actual page data.
           ShouldBePages = tango:scan_backward(P2, StreamNum, BackwardStartLPN,
                                               true)
       end || BackwardStartLPN <- lists:seq(1, NumPages)],
  
      ok.
  
negative.code:                    true             -> ok end,
                  {Proj2, tango:add_back_pointer(StreamNum, BackPs, LPN)}
          end,
      {_Px, _BackPs} = Res = lists:foldl(F, {Proj0, InitialBackPs}, Pages),
      %% io:format(user, "BackPs ~w\n", [_BackPs]),
      Res.
  
  scan_backward_test() ->
  