/**
   * Adds the {@code Runnable} and accompanying {@code Executor} to the list of listeners to
   * execute. If execution has already begun, the listener is executed immediately.
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
   * documentation.
   */