rintro ⟨σ, n⟩ σ' ⟨⟨⟩, ⟨⟩⟩
case hg
α : Type u_1
σ : Stream' α
⊢ ∀ (a : Stream' α × ℕ) (a' : Stream' α),
    (match a with
      | (σ, n) => σ = a' ∧ n = 0) →
      match
        match a with
        | (σ, 0) => (tail (tail σ), 0)
        | (σ, Nat.succ m) => (tail σ, m) with
      | (σ, n) => σ = tail (tail a') ∧ n = 0
case hg.mk.intro.refl.refl
α : Type u_1
σ✝ σ : Stream' α
⊢ match
    match (σ, 0) with
    | (σ, 0) => (tail (tail σ), 0)
    | (σ, Nat.succ m) => (tail σ, m) with
  | (σ_1, n) => σ_1 = tail (tail σ) ∧ n = 0