def p_postfix_expression3(self, p):
    ' postfix_expression : postfix_expression LBRACKET expression RBRACKET '
    p[0] = ArraySymbol(p[1], p[3])
postfix_expression : postfix_expression LBRACKET expression RBRACKET
def p_postfix_expression3(self, p):
    '  '
    p[0] = ArraySymbol(p[1], p[3])