∀ {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 fallback : α},
      (m₁.diff m₂ ⋯).getKeyD k fallback =
        if Std.DTreeMap.Internal.Impl.contains k m₂ = true then fallback else m₁.getKeyD k fallback