intro a b
case h.e_a.zero
α : Type
⊢ ∀ (lengths : Stream' ℕ) (a : Stream' α),
    ((Stream'.iterate (fun x => (x.1.tail, Stream'.drop x.1.head x.2)) (lengths, a)).get 0).2 =
      Stream'.drop (∑ j ∈ Finset.range 0, lengths j) a
case h.e_a.zero
α : Type
a : Stream' ℕ
b : Stream' α
⊢ ((Stream'.iterate (fun x => (x.1.tail, Stream'.drop x.1.head x.2)) (a, b)).get 0).2 =
    Stream'.drop (∑ j ∈ Finset.range 0, a j) b