rintro ⟨_, _⟩ i ⟨⟨⟩, ⟨⟩⟩
case hf
α✝ : Type u_1
σ : Stream' α✝
⊢ ∀ (a : Stream' α✝ × ℕ) (a' : ℕ),
    (match a with
      | (σ', c) => σ' = drop (a' + a' / 2) σ ∧ c = (a' + 1) % 2) →
      (match a with
        | (σ, snd) => head σ) =
        σ (a' + a' / 2)
case hf.mk.intro.refl.refl
α✝ : Type u_1
σ : Stream' α✝
i : ℕ
⊢ (match (drop (i + i / 2) σ, (i + 1) % 2) with
    | (σ, snd) => head σ) =
    σ (i + i / 2)