第20行访问栈顶(栈顶的索引为- 1)作为lua字符串，打印消息，然后从堆栈中删除该值。

Line 20 accesses the top of the stack (the top of the stack has an index of -1) as a Lua string, prints the message, and then removes the value from the stack.

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指令PULL或者PARSE PULL从栈顶取得一个字符串。

The instruction PULL or PARSE PULL take a string off the top of the stack.

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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.

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