code: func (q *Quadtree) FindKNearest(p *geo.Point, k int, maxDistance ...float64) []geo.Pointer {
  	return q.FindKNearestMatching(p, k, nil, maxDistance...)
  }
docs: // FindKNearest returns k closest Value/Pointer in the quadtree.
  // This function is thread safe. Multiple goroutines can read from
  // a pre-created tree.
  // This function allows defining a maximum distance in order to reduce search iterations.