{α : Type u} →
  {β : α → Type v} →
    {cmp : α → α → Ordering} →
      {γ : Sort w} →
        (f : Std.DTreeMap α β cmp → Std.DTreeMap α β cmp → γ) →
          (∀ (a b c d : Std.DTreeMap α β cmp), a.Equiv c → b.Equiv d → f a b = f c d) →
            Std.ExtDTreeMap α β cmp → Std.ExtDTreeMap α β cmp → γ