∀ {α : Type u} {instOrd : Ord α} {β : Type v} {m₁ m₂ : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α],
  m₁.WF →
    m₂.WF →
      ∀ {k : α},
        Std.DTreeMap.Internal.Impl.contains k m₂ = true → Std.DTreeMap.Internal.Impl.Const.get? (m₁.diff! m₂) k = none