intro i
case succ
m : ℕ
ih : ∀ (i : ℕ) (h : i < length (rangeF m)), getF (rangeF m) i h = { val := i, isLt := (_ : i < m) }
⊢ ∀ (i : ℕ) (h : i < length (rangeF (Nat.succ m))),
    getF (rangeF (Nat.succ m)) i h = { val := i, isLt := (_ : i < Nat.succ m) }
case succ
m : ℕ
ih : ∀ (i : ℕ) (h : i < length (rangeF m)), getF (rangeF m) i h = { val := i, isLt := (_ : i < m) }
i : ℕ
⊢ ∀ (h : i < length (rangeF (Nat.succ m))), getF (rangeF (Nat.succ m)) i h = { val := i, isLt := (_ : i < Nat.succ m) }