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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def step(self, actions, step_mul=None):
 """Apply actions, step the world forward, and return observations. Args: actions: A list of actions meeting the action spec, one per agent. step_mul: If specified, use this rather than the environment's default. Returns: A tuple of TimeStep namedtuples, one per agent. """

    if self._state == environment.StepType.LAST:
      return self.reset()

    skip = not self._ensure_available_actions
    self._parallel.run(
        (c.act, f.transform_action(o.observation, a, skip_available=skip))
        for c, f, o, a in zip(
            self._controllers, self._features, self._obs, actions))

    self._state = environment.StepType.MID
    return self._step(step_mul)