code: def consume(self, tokens):
          
          wait_time = 0.
          self.tokens -= tokens
          if self.tokens < 0:
              self._get_tokens()
          if self.tokens < 0:
              wait_time = -self.tokens / self.fill_rate
          return wait_time
docs: Consume tokens.
          Args:
              tokens (float): number of transport tokens to consume
          Returns:
              wait_time (float): waiting time for the consumer