public default R visitUnknown (Directive d, P p)

Visits an unknown directive. This can occur if the language evolves and new kinds of directive are added.

Parameters:
d    the directive to visit
p    a visitor-specified parameter

Returns:  a visitor-specified result

Exceptions:
UnknownDirectiveException    a visitor implementation may optionally throw this exception

@implSpec This implementation throws new UnknownDirectiveException(d, p).