@Override
    String toPendingString() {
      return callable.toString();
    }
  }

  @WeakOuter
  private final class CallableInterruptibleTask extends CombinedFutureInterruptibleTask<V> {
    private final Callable<V> callable;

    CallableInterruptibleTask(Callable<V> callable, Executor listenerExecutor) {
      super(listenerExecutor);
      this.callable = checkNotNull(callable);
    }