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.

youdao

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

如果你向堆栈压入数据2,4,6，然后弹出栈两次，你将按照顺序得到6与4。

If you push the Numbers 2, 4, and 6 onto a stack and then pop the stack twice, you will get 6 and 4, in that order.

youdao

现在，我的单元测试-向栈上压入2，4，6，然后从栈顶弹出两个元素以检验他们是否为6，4-失败了。

Now, my unit test -- which pushes 2, 4, and 6 onto the stack and pops the top two elements off to verify that they are 6 and 4 -- fails.

youdao

现在，我的单元测试-向栈上压入2，4，6，然后从栈顶弹出两个元素以检验他们是否为6，4-失败了。

Now, my unit test -- which pushes 2, 4, and 6 onto the stack and pops the top two elements off to verify that they are 6 and 4 -- fails.

youdao