/**
* Appends a formatter to the builder which will optionally format/parse.
* <p>
* This method has the same effect as appending each of the constituent
* parts directly to this builder surrounded by an {@link #optionalStart()} and
* {@link #optionalEnd()}.
* <p>
* The formatter will format if data is available for all the fields contained within it.
* The formatter will parse if the string matches, otherwise no error is returned.
*
* @param DateTimeFormatter $formatter the formatter to add, not null
* @return DateTimeFormatterBuilder this, for chaining, not null
*/
public function appendOptional(DateTimeFormatter $formatter)
{
$this->appendInternal($formatter->toPrinterParser(true));
return $this;
}