src/org/nutz/el/arithmetic/RPN.java
public Object calculate(Context context, Queue<Object> rpn) {
        ec.setContext(context);
        LinkedList<Object> operand = OperatorTree(rpn);
        return calculate(operand);
    }
public Object calculate(Context context, Queue<Object> rpn) {
        ec.setContext(context);
        LinkedList<Object> operand = OperatorTree(rpn);
        return calculate(operand);
    }
根据逆波兰表达式进行计算