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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_hierarchy_traversal_session(self, proxy):
 """Gets the ``OsidSession`` associated with the hierarchy traversal service. arg: proxy (osid.proxy.Proxy):
 a proxy return: (osid.hierarchy.HierarchyTraversalSession) - a ``HierarchyTraversalSession`` raise: NullArgument - ``proxy`` is ``null`` raise: OperationFailed - unable to complete request raise: Unimplemented - ``supports_hierarchy_traversal()`` is ``false`` *compliance: optional -- This method must be implemented if ``supports_hierarchy_traversal()`` is ``true``.* """

        if not self.supports_hierarchy_traversal():
            raise errors.Unimplemented()
        # pylint: disable=no-member
        return sessions.HierarchyTraversalSession(proxy=proxy, runtime=self._runtime)