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

对于输入的每一对字符串，你的程序应该按顺序生成所有的有效的进出栈操作列表，其中的每组操作都能使原字符串转变为目标字符串。

For each input pair, your program should produce a sorted list of valid sequences of I and o which produce the target word from the source word.

youdao

其概念是非常简单的：通过使用整个软件栈(操作系统、应用、配置数据等等)的虚拟映像来避免安装与配置上的冲突。

The concept is simple: avoid all installation and configuration hassles by using a virtual image of an entire software stack: operating system, application (s), configuration data, etc.

youdao