∀ {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) (h₂ : m₂.WF) {k : α},
  Std.DTreeMap.Internal.Impl.Const.get? (m₁.union m₂ ⋯ ⋯) k =
    (Std.DTreeMap.Internal.Impl.Const.get? m₂ k).or (Std.DTreeMap.Internal.Impl.Const.get? m₁ k)