docs: def v1_subtopic_list(request, response, kvlclient, fid, sfid):
      '''Retrieves a list of items in a subfolder.
  
      The route for this endpoint is:
      ``GET /dossier/v1/folder/<fid>/subfolder/<sfid>``.
  
      (Temporarily, the "current user" can be set via the
      ``annotator_id`` query parameter.)
  
      The payload returned is a list of two element arrays. The first
      element in the array is the item's content id and the second
      element is the item's subtopic id.
      '''
      path = urllib.unquote(fid) + '/' + urllib.unquote(sfid)
      try:
          items = []
          for it in new_folders(kvlclient, request).list(path):
              if '@' in it.name:
                  items.append(it.name.split('@'))
              else:
                  items.append((it.name, None))
          return items
      except KeyError:
          response.status = 404
          return []