cases i
n✝ n : Nat
i : Fin n
⊢ i.succ.iotaFind = (List.Index.map succ i.iotaFind).tail
case mk
n✝ n val✝ : Nat
isLt✝ : val✝ < n
⊢ ⟨val✝, isLt✝⟩.succ.iotaFind = (List.Index.map succ ⟨val✝, isLt✝⟩.iotaFind).tail