* @return a new, empty {@code TreeMap}
   */
  public static <C extends @Nullable Object, K extends C, V extends @Nullable Object>
      TreeMap<K, V> newTreeMap(@CheckForNull Comparator<C> comparator) {
    // Ideally, the extra type parameter "C" shouldn't be necessary. It is a
    // work-around of a compiler type inference quirk that prevents the
    // following code from being compiled:
    // Comparator<Class<?>> comparator = null;