code: func New(bound *geo.Bound, preallocateSize ...int) *Quadtree {
  	qt := &Quadtree{
  		Threshold: math.Max(bound.Width(), bound.Height()) / float64(1<<12),
  		bound:     bound,
  	}
  	if len(preallocateSize) == 1 {
  		qt.freeNodes = make([]node, preallocateSize[0], preallocateSize[0])
  
  	}
  	return qt
  }
docs: // New creates a new quadtree for the given bound. Added points
  // must be within this bound.