private Comparison comparison() {
        ArrayList<Expr> exprlist = new ArrayList<Expr>();
        ArrayList<CompOp> compopList = new ArrayList<CompOp>();
        Expr fisrtExpr = expr();
        while (lexer.token == Symbol.EQ
                || lexer.token == Symbol.NEQ || lexer.token == Symbol.NOT
                || lexer.token == Symbol.GT || lexer.token == Symbol.LT
                || lexer.token == Symbol.GE
                || lexer.token == Symbol.LE || lexer.token == Symbol.IN
                || lexer.token == Symbol.IS) {
            compopList.add(comp_op());
            exprlist.add(expr());
        }
        Comparison comp = new Comparison(fisrtExpr, compopList, exprlist);
        return comp;
    }