code: func (b *Backend) GroupTaskStates(groupUUID string, groupTaskCount int) ([]*tasks.TaskState, error) {
  	taskUUIDs, ok := b.groups[groupUUID]
  	if !ok {
  		return nil, NewErrGroupNotFound(groupUUID)
  	}
  
  	ret := make([]*tasks.TaskState, 0, groupTaskCount)
  	for _, taskUUID := range taskUUIDs {
  		t, err := b.GetState(taskUUID)
  		if err != nil {
  			return nil, err
  		}
  
  		ret = append(ret, t)
  	}
  
  	return ret, nil
  }
docs: // GroupTaskStates returns states of all tasks in the group