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.

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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.

youdao