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 ⊥.