∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {k : α} {fallback : β k}, k ∉ t₂ → (t₁ ∩ t₂).getD k fallback = fallback