∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.ExtTreeSet α cmp} [inst : Std.TransCmp cmp] [Std.LawfulEqCmp cmp],
  t ≠ ∅ → ∀ {km fallback : α}, t.maxD fallback = km ↔ km ∈ t ∧ ∀ k ∈ t, (cmp k km).isLE = true