∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering}
  {motive : Std.ExtDTreeMap α β cmp → Std.ExtDTreeMap α β cmp → Prop} (t₁ t₂ : Std.ExtDTreeMap α β cmp),
  (∀ (a b : Std.DTreeMap α β cmp), motive (Std.ExtDTreeMap.mk a) (Std.ExtDTreeMap.mk b)) → motive t₁ t₂