*     .transform((closer, result) -> result.get("userName"), directExecutor())
 *     .catching(DBException.class, e -> "no user", directExecutor())
 *     .finishToValueAndCloser(
 *         valueAndCloser -> this.userNameValueAndCloser = valueAndCloser, executor);
 *
 * // later
 * try { // get() will throw if the operation failed or was cancelled.
 *   UserName userName = userNameValueAndCloser.get();
 *   // do something with userName