anchor.diff:  %% 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 ->
  
positive.code:     % between zero and twice the counter plus one.
      elements(lists:seq(0, 2*Counter+1)).
  
  
negative.code: 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 ->
  