∀ {α : Type u_2} {β : Type u_1} {f : α → β} {b : β} {s : Stream' α} (motive : b ∈ Stream'.map f s → Prop)
  (x : b ∈ Stream'.map f s), (∀ (n : ℕ) (h : b = (Stream'.map f s).get n), motive ⋯) → motive x