About lex and yacc

[parag]

I saw the following code some where,  How come the following yacc rule
uses $4, which does not exist. Can you help me

internal_op_call_with_args
        : IPOINT '('
                {

                    if ((void*)$1 == (void*)CSample) {
                        fcSTF = 1;
                    }
                }
           expression_or_null_list ')'
                {

                    $$ = (long)getInternalOpPtr((void*)$1, (PT*)$4);
                }