∀ {α₁ α₂ β : Type w} [inst : Std.Iterator α₁ Id β] [inst_1 : Std.Iterator α₂ Id β] [Std.Iterators.Productive α₁ Id]
  [Std.Iterators.Productive α₂ Id] {it₁ : Std.Iter β} {it₂ : Std.Iter β} {n k : ℕ},
  Std.Iter.atIdxSlow? k it₁ = none →
    (∀ j < k, (Std.Iter.atIdxSlow? j it₁).isSome = true) →
      k ≤ n → Std.Iter.atIdxSlow? n (it₁.append it₂) = Std.Iter.atIdxSlow? (n - k) it₂