code: func NewExpiringClusteringIndex(expiration Minutes) *ClusteringIndex {
  	index := &ClusteringIndex{}
  	index.streetLevel = NewExpiringPointsIndex(Km(0.5), expiration)
  	index.cityLevel = NewExpiringCountIndex(Km(10), expiration)
  	index.worldLevel = NewExpiringCountIndex(Km(500), expiration)
  
  	return index
  }
docs: // NewExpiringClusteringIndex creates index that clusters the points at three levels with cell size 0.5, 5 and 500km and
  // expires them after expiration minutes.