* the function is not invoked). Example usage:
   *
   * {@snippet :
   * ListenableFuture<QueryResult> queryFuture = ...;
   * ListenableFuture<List<Row>> rowsFuture =
   *     transform(queryFuture, QueryResult::getRows, executor);
   * }
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the warnings the {@link MoreExecutors#directExecutor} documentation.
   *