simp [count, sum] at k
case zero
p : Fin 0 → Prop
inst : DecidablePred p
k : Fin (count p)
i : { i // p i }
⊢ decodeSubtype p k = i ↔ encodeSubtype p i = k
case zero
p : Fin 0 → Prop
inst : DecidablePred p
k : Fin 0
i : { i // p i }
⊢ decodeSubtype p k = i ↔ encodeSubtype p i = k