∀ {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 m₂ = true → (m₁.diff m₂ ⋯).getKey? k = none