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