[antlr-interest] translation of $x references in v3.0

[Matthew Ford]

I agree with Loring, and vote for just one prefix.  (the backwards
compatible switch sounds good too)
matthew

> In fact, I'd vote against having more than one special character
> for attributes in actions:  the only feature it provides the user is to
> allow duplicate use of names and it complicates the translation mechanism
> for the ANTLR backend developer.
>