@abc.abstractmethod<EOL><INDENT>def get_assignable_book_ids(self, book_id):<DEDENT>
return<EOL>
Gets a list of books including and under the given book node in which any comment can be assigned.

        :param book_id: the ``Id`` of the ``Book``
        :type book_id: ``osid.id.Id``
        :return: list of assignable book ``Ids``
        :rtype: ``osid.id.IdList``
        :raise: ``NullArgument`` -- ``book_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request

        *compliance: mandatory -- This method must be implemented.*