btw, here's a nice application of the deep option with factor: what is the
angle subtended by a circle of radius r that is a distance d from the
origin?

```
>>> t=Circle((d,0),r).tangent_lines((0,0))
>>> a=t[0].angle_between(t[1])
>>> simplify(a)
acos((d**4 - 3*d**2*r**2 + 2*r**4)/(d**2*(d**2 - r**2)))
>>> factor(_)
acos(d**4/(d**4 - d**2*r**2) - 3*d**2*r**2/(d**4 - d**2*r**2) +
2*r**4/(d**4 - d
**2*r**2))
>>> factor(_,deep=1)
acos(-(-d**2 + 2*r**2)/d**2)
>>> expand(_)
acos(1 - 2*r**2/d**2)
```