code: func chunkRectangles(slice rtree.Rectangles, numParts int64) []rtree.Rectangles {
  	parts := make([]rtree.Rectangles, numParts)
  	for i := int64(0); i < numParts; i++ {
  		parts[i] = slice[i*int64(len(slice))/numParts : (i+1)*int64(len(slice))/numParts]
  	}
  	return parts
  }
docs: // chunkRectangles takes a slice of rtree.Rectangle values and chunks it into `numParts` subslices.