def get_hierarchy_traversal_session(self):
raise Unimplemented()<EOL>
Gets the ``OsidSession`` associated with the hierarchy traversal service.

        return: (osid.hierarchy.HierarchyTraversalSession) - a
                ``HierarchyTraversalSession``
        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``.*