code: func (l *lifoProcessor) Concat(cnx redis.Conn, src, dest string) (err error) {
  	bytes, err := l.PullTo(cnx, src, dest, 0*time.Second)
  	if err == nil && bytes == nil {
  		err = redis.ErrNil
  	}
  
  	return
  }
docs: // Concat removes the first element from the source list and adds it to the end
  // of the destination list. ErrNil is returns when the source is empty.