∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α],
  t.WF → ∀ {k : α} {v : β k}, t.isEmpty = true → (Std.DTreeMap.Internal.Impl.insert! k v t).minKey? = some k