code: public String toKey() {
          return (tag == null ? EMPTY_STRING : tag) + KEY_DELIMITER + (indicator == null ? EMPTY_STRING : indicator) +
                  KEY_DELIMITER + subfieldIds;
      }
docs: A MARC field can be denoted by a key, independent of values.
  This key is a string, consisting of tag, indicator, subfield IDs, delimited by a dollar sign.
  
  @return the key of this MARC field