anchor.diff:  %% remains in the active set.
   handler(yield, Pid, #info{active = Active} = Info, _Opt) ->
       Lid = lid(Pid),
  -    debug("Process ~p yields.~n", [Lid]),
  +    ?debug("Process ~p yields.~n", [Lid]),
       NewActive = set_add(Active, Lid),
       Info#info{active = NewActive}.
   
  
positive.code: log(String, Args) ->
      io:format(String, Args).
  
  %% Display a debug message.
  %% debug(String) ->
  %%     io:format("(Debug)" ++ String).
  
  debug(String, Args) ->
      case ?DEBUG of
  	true -> io:format("(Debug)" ++ String, Args);
  	false -> ok
      end.
  
  %%%----------------------------------------------------------------------
  %%% Set interface
  
negative.code: 	false ->
  	    %% Remove process Next from the `active` set and run it.
  	    NewActive = set_remove(Active, Next),
  	    ?debug("Running process ~p.~n", [Next]),
  	    run(Next),
  	    %% Create new state.
  	    NewState = state_get_next(State, Next),
  