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