∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF → ∀ {k a : α}, (t.erase k).getKey? a = if cmp k a = Ordering.eq then none else t.getKey? a