peek() 应该保留栈顶的项目。
在碰到退出函数时,当前的活动函数就会返回,并释放栈顶元素。
When an exit address is encountered, the current active function has returned and the top element on the stack is discarded.
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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