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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 playbook(self):
 """Include the Playbook Module. .. Note:: Playbook methods can be accessed using ``tcex.playbook.<method>``. """

        if self._playbook is None:
            from .tcex_playbook import TcExPlaybook

            self._playbook = TcExPlaybook(self)
        return self._playbook