Why are you inheriting antlr.g? I found for my TAGS generator that inheriting wasn't flexible enough for me to identify definitions and uses. I copied antlr.g and then hacked away at it. When antlr.g changes between releases it amounts to the same amount of work to change it whether it's inherited or not.

I do agree that reportError and such might better be located outside of antlr.g so they can be reused.