In this example ``sign(a)`` is unevaluated; that is the reason for using `sign(a.n())`

```
>>> a = sqrt(-625 + 29*sqrt(5))*sqrt(-5 + sqrt(5))
>>> a1 = sqrtdenest(sqrt(_mexpand(a**2))); a1
-7*sqrt(5) + 55
>>> sign(a)
sign(sqrt(-625 + 29*sqrt(5))*sqrt(-5 + sqrt(5)))
>>> sign(a.n())
-1
```