∀ {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} {fallback : β},
        Std.DTreeMap.Internal.Impl.contains k m₂ = true →
          Std.DTreeMap.Internal.Impl.Const.getD (m₁.diff! m₂) k fallback = fallback