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