Bounded and unbounded are not antonyms

It is currently possible to create a meaningless set of assumptions:

neither
```
    Dummy(zero=True, bounded=False) nor
    Dummy(zero=True, unbounded=True)
```
raise an error.

Further discussion is in the commit involving NaN.

- [ ] clean up S.in_Q discussion
- [x] change unbounded to infinite
- [ ] get good reference for definition of bounded (abs(x) < oo) and finite (0 < abs(x) < oo)
- [ ] what's the right answer for wester M24 test?
This looks wrong and the old solution right. This equation is easy to solve by hand (note that `binomial(m, 2) = m*(m - 1)/2`).