docs: def labels_to_indexed_coref_values(content_objs, labels):
      '''[(content_id, FeatureCollection)] -> [Label] -> [({-1,1}, i, j)]
  
      where 0 <= i, j < len(content_objs).
      '''
      cids_to_idx = {}
      for i, (content_id, _) in enumerate(content_objs):
          cids_to_idx[content_id] = i
      idx = lambda cid: cids_to_idx[cid]
      labs = []
      for lab in labels:
          if lab.content_id1 in cids_to_idx and lab.content_id2 in cids_to_idx:
              labs.append((lab.value.value, idx(lab.content_id1), idx(lab.content_id2)))
      return labs