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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 get_install_dir(self):
 """ Returns the path to the cassandra source directory used by this node. """

        if self.__install_dir is None:
            return self.cluster.get_install_dir()
        else:
            common.validate_install_dir(self.__install_dir)
            return self.__install_dir