Prove that `power2` is strict. While one can base the proof on the "obvious" fact that `power2 n` is $2^n$, the latter is preferably proven using fixed point iteration.
```power2 0 = 1
First, we need to consider whether `⊥ - 1` is `⊥` or not. Using similar reasoning to the `⊥ + 1` case, we can determine that `⊥ - 1` is indeed `⊥`. From there, we know that `power2 ⊥` is `2 * power2 ⊥`. Since this recurses indefinitely, it is equivalent to `⊥`.