code: public DateTimeFormatterBuilder append(DateTimeFormatter formatter) {
          if (formatter == null) {
              throw new IllegalArgumentException("No formatter supplied");
          }
          return append0(formatter.getPrinter0(), formatter.getParser0());
      }
docs: Appends another formatter.
  <p>
  This extracts the underlying printer and parser and appends them
  The printer and parser interfaces are the low-level part of the formatting API.
  Normally, instances are extracted from another formatter.
  Note however that any formatter specific information, such as the locale,
  time-zone, chronology, offset parsing or pivot/default year, will not be
  extracted by this method.
  
  @param formatter  the formatter to add
  @return this DateTimeFormatterBuilder, for chaining
  @throws IllegalArgumentException if formatter is null or of an invalid type