anchor.diff:  -define(SERVER, ?MODULE).
   
   -record(state, {
  -          nodes = [{node(), alive}] :: [{node(), term()}],
  +          nodes = [{node(), joined}] :: [{node(), node_status()}],
             tasks = dict:new() :: dict:dict(task_name(), {schedule(), detail()})
            }).
   
  
positive.code: %%--------------------------------------------------------------------
  handle_call({add, TaskName, Schedule, Detail}, _From, State = #state{ tasks = Tasks }) ->
      %% TODO: receive a message from task worker
      case dict:find(TaskName, Tasks) of
          error ->
              NewTasks = dict:append(TaskName, {Schedule, Detail}, Tasks),
              io:format("adds a task: {~p, ~p, ~p}", [TaskName, Schedule, Detail]),
              {reply, ok, State#state{ tasks = NewTasks }};
          _ ->
              io:format("already task is registered: {~p, ~p, ~p}", [TaskName, Schedule, Detail]),
              {reply, {already_registered, TaskName}, State}
      end;
  handle_call({cancel, TaskName}, _From, State) ->
      %% TODO: receive a message from task worker
      NewTasks = dict:erase(TaskName, State#state.tasks),
  
negative.code: handle_call({members}, _From, State) ->
      {reply, State#state.nodes, State};
  handle_call(Request, _From, State) ->
      {reply, {Request, bad_request}, State}.
  
  %%--------------------------------------------------------------------
  %% @private
  