<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start(self):
 """ Starts the thread pool. Does nothing if the pool is already started. """

        if not self._done_event.is_set():
            # Stop event not set: we're running
            return

        # Clear the stop event
        self._done_event.clear()

        # Compute the number of threads to start to handle pending tasks
        nb_pending_tasks = self._queue.qsize()
        if nb_pending_tasks > self._max_threads:
            nb_threads = self._max_threads
            nb_pending_tasks = self._max_threads
        elif nb_pending_tasks < self._min_threads:
            nb_threads = self._min_threads
        else:
            nb_threads = nb_pending_tasks

        # Create the threads
        for _ in range(nb_pending_tasks):
            self.__nb_pending_task += 1
            self.__start_thread()
        for _ in range(nb_threads - nb_pending_tasks):
            self.__start_thread()