* one input completes with result X and another later with result Y, for Y to come before X in
   * the output future list. (Such races are impossible to solve without global synchronization of
   * all future completions. And they should have little practical impact.)
   *
   * <p>Cancelling a delegate future propagates to input futures once all the delegates complete,

//          except operations referenced in `inputs`. These operations
//          must have a single output (these operations are typically
//          placeholders created for the sole purpose of representing
//          an input. We can relax this constraint if there are
//          compelling use cases).
//          - If an array is given (`num_opers` >= 0), all operations