∀ {m₁ : Type u_1 → Type u_2} {m₂ : Type u_4 → Type u_3} {α₁ ω₁ : Type u_1} {α₂ ω₂ : Type u_4}
  (F : m₁ (α₁ × ω₁) ≃ m₂ (α₂ × ω₂)) (f : m₁ (α₁ × ω₁)), WriterT.mk (F.symm (WriterT.mk (F f))) = WriterT.mk f