*
   * <p>After calling this method, you may not call {@link #finishToFuture()}, {@link
   * #finishToValueAndCloser(ValueAndCloserConsumer, Executor)}, or any other derivation method on
   * the original {@code ClosingFuture} instance.
   *
   * @param function transforms the value of this step to the value of the derived step
   * @param executor executor to run the function in
   * @return the derived step