Create a TravelVia VerbRule merely so we can supply a verbPhrase. (The parser looks for subclasses of each specific Action class to find its verb phrase, since the language-specific Action definitions are always in the language module's 'grammar' subclasses. We don't need an actual grammar rule, since this isn't an input-able verb, so we merely need to create a regular subclass in order for the verbPhrase to get found.)

VerbRule(TravelVia)      // original source text
grammar VerbRule(TravelVia) :   VerbProduction      // after macro expansion