Add an item to a subfolder.

        The format of ``content_id`` and ``subtopic_id`` is
        unspecified. It is application specific.

        If ``ann_id`` is set, then the item is owned by the given user.
        Otherwise, the item is owned and viewable by all anonymous
        users.

        :param str folder_id: Folder id
        :param str subfolder_id: Folder id
        :param str content_id: content identifier
        :param str subtopic_id: subtopic identifier
        :param str ann_id: Username

def add_item(self, folder_id, subfolder_id, content_id, subtopic_id=None,
                 ann_id=None):
        '''Add an item to a subfolder.

        The format of ``content_id`` and ``subtopic_id`` is
        unspecified. It is application specific.

        If ``ann_id`` is set, then the item is owned by the given user.
        Otherwise, the item is owned and viewable by all anonymous
        users.

        :param str folder_id: Folder id
        :param str subfolder_id: Folder id
        :param str content_id: content identifier
        :param str subtopic_id: subtopic identifier
        :param str ann_id: Username
        '''
        self.assert_valid_folder_id(folder_id)
        self.assert_valid_folder_id(subfolder_id)
        ann_id = self._annotator(ann_id)
        folder_cid = self.wrap_folder_content_id(ann_id, folder_id)
        subfolder_sid = self.wrap_subfolder_subtopic_id(subfolder_id)

        if self.store.get(folder_cid) is None:
            raise KeyError(folder_id)

        lab = Label(folder_cid, content_id,
                    ann_id, CorefValue.Positive,
                    subtopic_id1=subfolder_sid,
                    subtopic_id2=subtopic_id)
        self.label_store.put(lab)
        logger.info('Added subfolder item: %r', lab)