∀ {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 m₁ = false → (m₁.diff! m₂).getKey? k = none