pumping-lemma - 为什么 PDA 的抽引引理可以抽空？

Question

1. 对于每个i ≥ 0,uv^ixy^iz ∈ A,
2. |vy| > 0， 和
3. |vxy| ≤ p.

对于 2，如果我们抽空 uvxyz，我们会得到 uxz，但它会违反 2。因为 |vy| = 0。

我在很多地方都看到了这个例子，我在哪里理解错了？

Answer 1

Ok i got this. After a second thought, I think it meancs |vy| is non empty string, yet when they are pumped down, they repeat 0 times which make them disappear, but v y themselves are not defined empty.

Sorry for the stupid question.