public static Exp parse(Tokenizer t) throws ParseException {
        t.gotoEnd();
        return parseExp(t);
    }

    private static Exp parseExp(Tokenizer t) throws ParseException {
        Exp term = parseTerm(t);
        if (!t.done()) {
            Exp exp;
            if (t.current().isTheSymbol('+')) {
                t.previous();
                exp = parseExp(t);
                return new BinExp(exp, '+', term);
            } else if (t.current().isTheSymbol('-')) {
                t.previous();
                exp = parseExp(t);
                return new BinExp(exp, '-', term);
            } else {
                return term;
            }
        } else {
            return term;
        }
    }

    private static Exp parseTerm(Tokenizer t) throws ParseException {
        if (t.current().isTheSymbol(')')) {
            t.previous();
            Exp e = parseExp(t);
            if (t.current().isTheSymbol('(')) {
                t.previous();
                return e;
            } else {
                throw new ParseException("( expected");
            }
        } else if (t.current().isNumber()) {
            int v = t.current().number();
            t.previous();
            return new LitExp(v);
        } else {
            throw new ParseException("bracket or number expected");
        }
    }