[antlr-interest] Problem with rewrite rules with optional non-terminals

[David Jameson]

I have the following grammar fragment

refTag :
                 LSQUARE a=expression (COMMA b=expression)? RSQUARE
                    {
                          //stuff

                    } -> ^(REFTAG $a  $b)

                 ;


However, if I feed in source that does NOT have the optional comma  
expression clause, the parser crashes because it blindly tries to  
dereference  the stream_b object.

How can I test whether there was an optional expression and control  
the rewrite rule accordingly to include the $b or not?

Thanks,
D