docs: private HashMap<String, DBIDs> multipleAssignment(Relation<?> data) {
      HashMap<String, DBIDs> labelMap = new HashMap<>();
  
      for(DBIDIter iditer = data.iterDBIDs(); iditer.valid(); iditer.advance()) {
        String[] labels = data.get(iditer).toString().split(" ");
        for(String label : labels) {
          assign(labelMap, label, iditer);
        }
      }
      return labelMap;
    }