protected Object computeNumber(Integer left, String op, Integer right) {
		int a = left.intValue();
		int b = right.intValue();
		if ("+".equals(op)) {
			return a + b;
		} else if ("-".equals(op)) {
			return a - b;
		} else if ("*".equals(op)) {
			return a * b;
		} else if ("/".equals(op)) {
			return a / b;
		} else if ("%".equals(op)) {
			return a % b;
		} else if ("==".equals(op)) {
			return a == b;
		} else if (">".equals(op)) {
			return a > b;
		} else if ("<".equals(op)) {
			return a < b;
		}
		throw new StoneException("bad operator", this);
	}