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