anchor.diff:  next_state(S, _V, {call, ?SERVER, stream_filename, [_StreamID]}) ->
       S.
   
  +% define the conditions needed to be
  +% met in order for a test to pass
   postcondition(S, {call, ?SERVER, register_stream, [StreamName]}, Result) ->
       case proplists:is_defined(StreamName,S#state.streams) of
           true ->
  
positive.code:                 application:start(?APP),
                  {History,State,Result} = run_commands(?MODULE, Cmds),
                  application:stop(?APP),
                  ?WHENFAIL(
                      io:format("\n\nHistory: ~w\n\nState: ~w\n\nResult: ~w\n\n",
                      [History,State,Result]),
  
negative.code: %% Statem callbacks
  %%-----------------------------------------------------------------------------
  
  % initialize the state machine
  initial_state() ->
      #state{streams = [], counter = 0}.
  
  % define the commands to test
  command(S) ->
      oneof([{call, ?SERVER, register_stream, [streamname()]},
             {call, ?SERVER, stream_filename, [streamid(S)]}]).
  
  % define when a command is valid
  precondition(_S, _command) ->
      true. % All preconditions are valid
  
  % define the state transitions triggered
  % by each command
  next_state(S, _V, {call, ?SERVER, register_stream, [Stream]}) ->
      case proplists:is_defined(Stream, S#state.streams) of
          true ->
  