@abc.abstractmethod<EOL><INDENT>def get_group_session(self):<DEDENT>
return<EOL>
Gets the session for retrieving gropup memberships.

        :return: a ``GroupSession``
        :rtype: ``osid.resource.GroupSession``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_group()`` is ``false``

        *compliance: optional -- This method must be implemented if
        ``supports_group()`` is ``true``.*