∀ {α : Type u} {cmp : α → α → Ordering} {t : Std.TreeSet.Raw α cmp} [Std.TransCmp cmp] [Std.LawfulEqCmp cmp],
  t.WF → t.isEmpty = false → ∀ {km fallback : α}, t.maxD fallback = km ↔ km ∈ t ∧ ∀ k ∈ t, (cmp k km).isLE = true