### Derivative of O(x^0)

Consider a power series

```
R.<x> = PowerSeries(SR)
f = 1 + O(x^2)
f.derivative()
```

This gives us `O(x^1)`

as we would expect. However, if we do

```
f = O(x^0)
f.derivative()
```

we get `O(x^-1)`

instead of `O(x^0)`

again. Is this a bug or am I missing something?