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