docs: def create_label(self, label, doc=None, callback=dummy_progress_cb):
          """
          Create a new label
  
          Arguments:
              doc --- first document on which the label must be added (required
                      for now)
          """
          if doc:
              clone = doc.clone()  # make sure it's serializable
          r = self.index.create_label(label, doc=clone)
          return r