∀ {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},
  Std.DTreeMap.Const.get! (t₁ ∪ t₂) k = Std.DTreeMap.Const.getD t₂ k (Std.DTreeMap.Const.get! t₁ k)