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