code: func NewExpiringPointsIndex(resolution Meters, expiration Minutes) *PointsIndex {
  	currentPosition := make(map[string]Point)
  
  	newExpiringSet := func() interface{} {
  		set := newExpiringSet(expiration)
  
  		set.OnExpire(func(id string, value interface{}) {
  			point := value.(Point)
  			delete(currentPosition, point.Id())
  		})
  
  		return set
  	}
  
  	return &PointsIndex{newGeoIndex(resolution, newExpiringSet), currentPosition}
  }
docs: // NewExpiringPointsIndex creates new PointIndex that expires the points in each cell after expiration minutes.