- Make `flöb_pre` and `flöb` local to the proof.
- The metavariables `Ψ` are used for predicates, so use a `Q` here.

This follows the proof at https://en.wikipedia.org/wiki/L%C3%B6b's_theorem#Proof_of_L%C3%B6b's_theorem

Fixes #156

Fixes #177