have : ¬ i.succ < n.succ := λ h => absurd (Nat.lt_of_succ_lt_succ h) hnlt
α✝ α : Type u
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
acc : Array α
hnlt : ¬i < n
⊢ ofFn.go f (Nat.succ i) acc = ofFn.go (f ∘ Fin.succ) i acc
α✝ α : Type u
n : Nat
f : Fin (Nat.succ n) → α
i : Nat
acc : Array α
hnlt : ¬i < n
this : ¬Nat.succ i < Nat.succ n
⊢ ofFn.go f (Nat.succ i) acc = ofFn.go (f ∘ Fin.succ) i acc