∀ {α : Type u} {instOrd : Ord α} {β : Type v} {m₁ m₂ : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α]
  [inst : Inhabited β],
  m₁.WF →
    m₂.WF →
      ∀ {k : α},
        Std.DTreeMap.Internal.Impl.contains k m₁ = false →
          Std.DTreeMap.Internal.Impl.Const.get! (m₁.inter! m₂) k = default