code: func NewClusteringIndex() *ClusteringIndex {
  	index := &ClusteringIndex{}
  	index.streetLevel = NewPointsIndex(Km(0.5))
  	index.cityLevel = NewCountIndex(Km(10))
  	index.worldLevel = NewCountIndex(Km(500))
  
  	return index
  }
docs: // NewClusteringIndex creates index that clusters the points at three levels with cell size 0.5, 5 and 500km.
  // Useful for creating maps.