* the discussion in the {@link ListenableFuture#addListener} documentation. All its warnings
   * about heavyweight listeners are also applicable to heavyweight functions passed to this method.
   * (Specifically, {@code directExecutor} functions should avoid heavyweight operations inside
   * {@code AsyncClosingFunction.apply}. Any heavyweight operations should occur in other threads

   * the discussion in the {@link ListenableFuture#addListener} documentation. All its warnings
   * about heavyweight listeners are also applicable to heavyweight functions passed to this method.
   * (Specifically, {@code directExecutor} functions should avoid heavyweight operations inside
   * {@code AsyncClosingFunction.apply}. Any heavyweight operations should occur in other threads