An example where the sign can't be decided (because it's a zero that can't be calculated with precision):

```
>>> eq
-sqrt(10 + 6*sqrt(3)) + sqrt(1 + sqrt(3)) + sqrt(3 + 3*sqrt(3))
>>> eq.n()
-0.e-122
>>> _._prec
1
```

So the sign of that expression just comes back as `sign(eq)`.