anchor.diff:                    Unwritten  :: [byte_sequence()]
                ) -> {ok, Bytes :: binary(), Csum :: binary()} |
                     eof | 
  -                  {error, bad_csum} |
  +                  {error, bad_checksum} |
                     {error, partial_read} |
                     {error, not_written} |
                     {error, Other :: term() }.
  
positive.code:                 {error, Bad} ->
                      lager:error("Bad checksum; got ~p, expected ~p",
                                  [Bad, Ck]),
                      {error, bad_csum};
                  TaggedCsum ->
                      {ok, Bytes, TaggedCsum};
                  %% XXX FIXME: Should we return something other than
  
negative.code:         {error, {bad_csum, Bad}} ->
              lager:error("Bad checksum; client sent ~p, we computed ~p",
                          [ClientCsum, Bad]),
              {{error, bad_checksum}, Err + 1, U};
          TaggedCsum ->
              case handle_write(FHd, FHc, F, TaggedCsum, EofP, Data, U) of
                  {ok, NewU1} ->
  