docs: protected Task addTask(final long first, final Runnable runnable,
                             final long period, boolean relative) {
  
          if (this.isShutdown) {
              throw new IllegalStateException("shutdown");
          }
  
          final Task task = new Task(first, runnable, period, relative);
  
          // sychronized
          this.taskQueue.addTask(task);
  
          // sychronized
          this.restart();
  
          return task;
      }