∀ {m : Type u_1 → Type u_2} {α : Type u_3} {β : Type u_1} [inst : Monad m] [LawfulMonad m] (o : Option α)
  (f : (a : α) → a ∈ o → β → β) (b : β),
  (forIn' o b fun a m_1 b => pure (ForInStep.yield (f a m_1 b))) = pure (o.pelim b fun a h => f a h b)