∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t₁.WF → t₂.WF → ∀ {k : α}, (t₁ \ t₂).getKey? k = if t₂.contains k = true then none else t₁.getKey? k