First Order Logic

- [ ] Make sure tests pass
- [x] Use Symbol everywhere, not strings (or Constant or Variable or whatever)
- [ ] Add docstring with doctest to every class and function (including and especially abstract classes, although the abstract classes do not need to have doctests if that does not make sense). Major methods of classes should have this as well. 
- [x] Address https://github.com/sympy/sympy/pull/7608#discussion_r15020433
- [x] Address https://github.com/sympy/sympy/pull/7608#discussion_r15247231
- [ ] Address https://github.com/sympy/sympy/pull/7608/files#discussion_r15974116
- [ ] https://github.com/sympy/sympy/pull/7608/files#discussion_r15974291
- [x] Use a single to_cnf and to_dnf and use dispatching to distinguish the two. 
I see. But the convention in Python is already that things that start with an underscore are private, so this looks odd. Maybe use an underscore after the variable name. I think `X_domain` is fine too, though.