have hi' : i ≠ as.size - 1 := Nat.ne_of_lt <| size_del as k ▸ hi
α : Type u_1
as : Array α
k : Fin as.size
i : Nat
hi : i < (as.del k).size
⊢ (as.del k)[i] = if i = ↑k then as[as.size - 1] else as[i]
α : Type u_1
as : Array α
k : Fin as.size
i : Nat
hi : i < (as.del k).size
hi' : i ≠ as.size - 1
⊢ (as.del k)[i] = if i = ↑k then as[as.size - 1] else as[i]