∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.ExtDTreeMap α β cmp} [inst : Std.TransCmp cmp]
  {k : α}, (t.erase k).maxKey? = t.maxKey? ↔ ∀ {km : α}, t.maxKey? = some km → ¬cmp k km = Ordering.eq