docs: private HashMap<String, DBIDs> singleAssignment(Relation<?> data) {
      HashMap<String, DBIDs> labelMap = new HashMap<>();
  
      for(DBIDIter iditer = data.iterDBIDs(); iditer.valid(); iditer.advance()) {
        final Object val = data.get(iditer);
        String label = (val != null) ? val.toString() : null;
        assign(labelMap, label, iditer);
      }
      return labelMap;
    }