现在我可以为pop方法编写行为，即确保当栈在逻辑上为空时，抛出一个异常。
Now I can code the behavior for the pop method, which ensures that if the stack is logically empty, an exception will be thrown.
如果再次调用pop，那么应该返回foo，然后栈为空(假设在添加这两个对象之前栈中没有对象)。
If you called pop again, then foo should be returned and the stack should be empty (assuming there wasn't anything in it before you added the two objects).
在这种情况下，如果push没有被调用(或者从逻辑上讲，栈为空)的情况下确保pop抛出一个异常，则意味着栈有一个状态。
In this case, ensuring that pop throws an exception if push hasn't been called (or logically, if the stack is empty) means that the stack has a state.
下推格值自动机以终态与以空栈两种接受词语言的方式是等价的。
The equivalence of acceptance words by PDLA with final state and with empty stack is proved.
如果栈为空时抛出。
如果栈为空时抛出。
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