rcases n with ⟨⟩|⟨⟩|⟨⟩|_ <;> rfl
α : Type ?u.21002
σ✝ : Stream' α
n : ℕ
σ : Stream' ℕ
⊢ (match (σ, n - 2) with
      | (σ, 0) => (head σ, tail (tail σ), n - 2)
      | (σ, Nat.succ m) => (head σ, tail σ, m)).fst =
    head σ
no goals