rintro ⟨σ, n⟩ σ' ⟨⟨⟩, ⟨⟩⟩
case hf
α : Type u_1
σ : Stream' α
⊢ ∀ (a : Stream' α × ℕ) (a' : Stream' α),
    (match a with
      | (σ, n) => σ = a' ∧ n = 0) →
      head a.fst = head a'
case hf.mk.intro.refl.refl
α : Type u_1
σ✝ σ : Stream' α
⊢ head (σ, 0).fst = head σ