default <ST>Xor<ST,T> toXor(final ST secondary){
  final Optional<T> o=toOptional();
  return o.isPresent() ? Xor.primary(o.get()) : Xor.secondary(secondary);
}

default < ST > Xor < ST , T > toXor ( final ST secondary ) { final Optional < T > o = toOptional ( ) ; return o . isPresent ( ) ? Xor . primary ( o . get ( ) ) : Xor . secondary ( secondary ) ; }
convert to an xor where the secondary value will be used if no primary value is present
