在传统的语言中，这样的递归需要连续不断的入栈操作，以维护调用的历史；而在Scheme中，却不是这样。
In traditional languages, this recursion eats away at the stack to maintain a history of the calls; in Scheme, it's different.
当它是一个操作符时，从栈中弹出两个数进行运算，并且将运算结果压入栈中。
When it is an operator, pop two Numbers from the stack, do the calculation, and push back the result.
当它是一个操作符时，从栈中弹出两个数进行运算，并且将运算结果压入栈中。
When it is an operator, pop two Numbers from the stack, do the calculation, and push back the result.
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