rfl
case h.e_a.succ
α : Type
pn : ℕ
hpn :
  ∀ (lengths : Stream' ℕ) (a : Stream' α),
    ((Stream'.iterate (fun x => (x.1.tail, Stream'.drop x.1.head x.2)) (lengths, a)).get pn).1.head = lengths pn
a : Stream' ℕ
b : Stream' α
⊢ (a, b).1.tail pn = a (pn + 1)
no goals