def get_agency_hierarchy_session(self):
raise Unimplemented()<EOL>
Gets the session traversing agency hierarchies.

        return: (osid.authentication.AgencyHierarchySession) - an
                ``AgencyHierarchySession``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_agency_hierarchy()`` is
                ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_agency_hierarchy()`` is ``true``.*