def subfolders(self, folder_id, ann_id=None):
        self.assert_valid_folder_id(folder_id)
        ann_id = self._annotator(ann_id)
        folder_cid = self.wrap_folder_content_id(ann_id, folder_id)
        if self.store.get(folder_cid) is None:
            raise KeyError(folder_id)
        all_labels = self.label_store.directly_connected(folder_cid)
        return nub(la.subtopic_for(folder_cid) for la in all_labels)
Yields an unodered generator of subfolders in a folder.
dataset/ETHPy150Open dossier/dossier.web/dossier/web/label_folders.py/Folders.subfolders