∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {m₁ m₂ : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Std.LawfulEqOrd α],
  m₁.WF →
    m₂.WF →
      ∀ {k : α} [inst_1 : Inhabited (β k)],
        Std.DTreeMap.Internal.Impl.contains k m₂ = false → (m₁.union! m₂).get! k = m₁.get! k