为何下面的代码,不报索引超限错误呢?在推导式中,当x=0时,line只有一个元素,line[x+1]不久超下标了吗?
```python
def yahui():
line=[1]
while True:
yield line
line=[1]+[line[x]+line[x+1] for x in range(len(line)-1)] + [1]
yh = yahui()
n = 0
while n < 3:
print(next(yh))
n += 1
```