∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap α β cmp} [Std.TransCmp cmp] [Std.LawfulEqCmp cmp]
  [inst : Inhabited α], t.isEmpty = false → ∀ {km : α}, t.maxKey! = km ↔ km ∈ t ∧ ∀ k ∈ t, (cmp k km).isLE = true