code: def update(self, loss, force=False):
      
      do_update = super(CombineCurriculum, self).update(loss, force)
      if not do_update:
        return False
  
      if self._curr_length < self._max_length:
        self._curr_length += 1
      elif self._curr_nesting < self._max_nesting:
        self._curr_nesting += 1
      else:
        self._set_new_task = False
  
      if self._set_new_task:
        tf.logging.info("New level: (length=%d, nesting=%d)",
                        self._curr_length, self._curr_nesting)
      return self._set_new_task
docs: Increments level difficulty (length and nesting) by 1 until maximum.