code: func (i *InParallel) Add(task func() error) {
  	i.wg.Add(1)
  
  	go func() {
  		defer i.wg.Done()
  		err := task()
  		if err != nil {
  			i.pool.Put(err)
  		}
  	}()
  }
docs: // Add runs the specified task in parallel and adds it to the waitGroup.