∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap α β cmp} [Std.TransCmp cmp]
  {k fallback : α}, (t₁ \ t₂).getKeyD k fallback = if k ∈ t₂ then fallback else t₁.getKeyD k fallback