let i+1 := i
k✝ : Nat
α : Type u_1
f : Fin k✝ → α
i j k : Nat
h : i + j = k✝
hk : k < (ofFn.go f i j h).length
⊢ (ofFn.go f i j h).get ⟨k, hk⟩ = f ⟨j + k, ⋯⟩
k✝ : Nat
α : Type u_1
f : Fin k✝ → α
i✝ j k i : Nat
h : i + 1 + j = k✝
hk : k < (ofFn.go f (i + 1) j h).length
⊢ (ofFn.go f (i + 1) j h).get ⟨k, hk⟩ = f ⟨j + k, ⋯⟩