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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def items(self, folder_id, subfolder_id, ann_id=None):
        '''Yields an unodered generator of items in a subfolder.

        The generator yields items, which are represented by a tuple
        of ``content_id`` and ``subtopic_id``. The format of these
        identifiers is unspecified.

        By default (with ``ann_id=None``), subfolders are shown for all
        anonymous users. Optionally, ``ann_id`` can be set to a username,
        which restricts the list to only subfolders owned by that user.

        :param str folder_id: Folder id
        :param str subfolder_id: Subfolder id
        :param str ann_id: Username
        :rtype: generator of ``(content_id, subtopic_id)``
        '''
        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)
        ident = (folder_cid, subfolder_sid)

        if self.store.get(folder_cid) is None:
            raise KeyError(folder_id)
        for lab in self.label_store.directly_connected(ident):
            cid = lab.other(folder_cid)
            subid = lab.subtopic_for(cid)
            yield (cid, subid)