∀ {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} [inst_1 : Inhabited (β k)], (t₁ ∪ t₂).get! k = t₂.getD k (t₁.get! k)