system_sentence : SYSTEM VAR
                           | SYSTEM VAR LPAREN features RPAREN

def p_system_sentence(self, t):
        """system_sentence : SYSTEM VAR
                           | SYSTEM VAR LPAREN features RPAREN"""

        if len(t) == 3:
            t[0] = system(t[2], reference=True, line=t.lineno(1))
        else:
            t[0] = system(t[2], t[4], line=t.lineno(1))