code: public final ZMonitor start()
      {
          if (started) {
              System.out.println("ZMonitor: Unable to start while already started.");
              return this;
          }
          agent.send(START);
          agent.recv();
          started = true;
          return this;
      }
docs: Starts the monitoring.
  Event types have to be added before the start, or they will take no effect.
  @return this instance.