* executor promptly, or else the returned {@code ListenableFuture} may fail to work. The task's
   * execution consists of blocking until the input future is {@linkplain Future#isDone() done}, so
   * each call to this method may claim and hold a thread for an arbitrary length of time. Use of
   * bounded executors or other executors that may fail to execute a task promptly may result in
   * deadlocks.
   *