∀ {σ : Sort u_1} {α : Type u_2} (f : σ → α → σ) (x : σ) (x_1 : Batteries.RBNode.Stream α),
  Batteries.RBNode.Stream.foldl f x x_1 =
    match x, x_1 with
    | b, Batteries.RBNode.Stream.nil => b
    | b, Batteries.RBNode.Stream.cons v r tail =>
      Batteries.RBNode.Stream.foldl f (Batteries.RBNode.foldl f (f b v) r) tail