code: func (v Vector) DropOthers(id ShortID) Vector {
  	for i, c := range v.Counters {
  		if c.ID == id {
  			v.Counters = v.Counters[i : i+1]
  			return v
  		}
  	}
  	return Vector{}
  }
docs: // DropOthers removes all counters, keeping only the one with given id. If there
  // is no such counter, an empty Vector is returned.