no goals
α : Type u
β : Type v
δ : Type w
a : α
l₁ l₂ : List α
s : Stream' α
⊢ a :: l₁ ++ l₂ ++ₛ s = a :: l₁ ++ₛ (l₂ ++ₛ s)
rw [List.cons_append, cons_append_stream, cons_append_stream, append_append_stream l₁]