def get_catalog_hierarchy_session(self, proxy):
        """Gets the catalog hierarchy traversal session.

        arg:    proxy (osid.proxy.Proxy): proxy
        return: (osid.cataloging.CatalogHierarchySession) - a
                ``CatalogHierarchySession``
        raise:  NullArgument - ``proxy`` is null
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_catalog_hierarchy()`` is
                ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_catalog_hierarchy()`` is ``true``.*

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