∀ {α : Type u_1} {β : Type u_2} {cmp : α → α → Ordering}
  (motive : (t : Std.ExtTreeMap α β cmp) → t ≠ ∅ → t.inner = ∅ → Prop) (t : Std.ExtTreeMap α β cmp) (h : t ≠ ∅)
  (x : t.inner = ∅), motive t h x