∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {m₁ m₂ : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  (h₁ : m₁.WF),
  m₂.WF →
    ∀ {k : α},
      Std.DTreeMap.Internal.Impl.contains k m₁ = false → Std.DTreeMap.Internal.Impl.contains k (m₁.diff m₂ ⋯) = false