code: public DateTimeParser toParser() {
          Object f = getFormatter();
          if (isParser(f)) {
              InternalParser ip = (InternalParser) f;
              return InternalParserDateTimeParser.of(ip);
          }
          throw new UnsupportedOperationException("Parsing is not supported");
      }
docs: Internal method to create a DateTimeParser instance using all the
  appended elements.
  <p>
  Most applications will not use this method.
  If you want a parser in an application, call {@link #toFormatter()}
  and just use the parsing API.
  <p>
  Subsequent changes to this builder do not affect the returned parser.
  
  @throws UnsupportedOperationException if parsing is not supported