code: func (d *BoundDistributor) Cancel(_ *dm.Quest_Desc, tok distributor.Token) (err error) {
  	d.Lock()
  	defer d.Unlock()
  	if tsk, ok := d.tasks[tok]; ok {
  		tsk.done = true
  		tsk.abnorm = &dm.AbnormalFinish{
  			Status: dm.AbnormalFinish_CANCELLED,
  			Reason: "cancelled via Cancel()"}
  	} else {
  		err = fmt.Errorf("MISSING task %q", tok)
  	}
  	return
  }
docs: // Cancel implements distributor.D