if (cause instanceof Error) {
      throw new ExecutionError((Error) cause);
    }
    /*
     * It's an Exception. (Or it's a non-Error, non-Exception Throwable. From my survey of such
     * classes, I believe that most users intended to extend Exception, so we'll treat it like an
     * Exception.)
     */
    throw new UncheckedExecutionException(cause);
  }

  /*

   * {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   *
   * <p><b>Note:</b> The returned view performs slightly better when {@code set1} is the smaller of
   * the two sets. If you have reason to believe one of your sets will generally be smaller than the
   * other, pass it first. Unfortunately, since this method sets the generic type of the returned