code: func RemoveOutlierPointersByQuadkey(pointers []geo.Pointer, level, threshold int) []*clustering.Cluster {
  
  	buckets := make(map[int64][]geo.Pointer)
  	for _, p := range pointers {
  		key := p.Point().Quadkey(level)
  
  		buckets[key] = append(buckets[key], p)
  	}
  
  	clusters := make([]*clustering.Cluster, 0, len(buckets))
  	for _, b := range buckets {
  		if len(b) >= threshold {
  			clusters = append(clusters, clustering.NewCluster(b...))
  		}
  	}
  
  	return clusters
  }
docs: // RemoveOutlierPointersByQuadkey will bucket all pointers by quad key (defined by the level)
  // and remove the buckets with less than threshold pointers. The buckets become the resulting point_clustering.Clusters.