* that use {@code comparator} to determine the least and greatest elements.
   */
  /*
   * TODO(cpovirk): Change to Comparator<? super B> to permit Comparator<@Nullable ...> and
   * Comparator<SupertypeOfB>? What we have here matches the immutable collections, but those also
   * expose a public Builder constructor that accepts "? super." So maybe we should do *that*
   * instead.
   */