code: func (points *PointsIndex) PointsWithin(point Point, distance Meters, accept func(p Point) bool) []Point {
  
  	d := int(distance / points.index.resolution)
  
  	if d == 0 {
  		d = 1
  	}
  
  	idx := cellOf(point, points.index.resolution)
  
  	nearbyPoints := make([]Point, 0)
  	nearbyPoints = getPointsAppend(nearbyPoints, points.index.get(idx.x-d, idx.x+d, idx.y-d, idx.y+d), accept)
  
  	// filter points which longer than maxDistance away from point.
  	withinPoints := make([]Point, 0)
  	for _, nearbyPoint := range nearbyPoints {
  		if Distance(point, nearbyPoint) < distance {
  			withinPoints = append(withinPoints, nearbyPoint)
  		}
  	}
  
  	return withinPoints
  }
docs: // PointsWithin returns all points with distance of point that match the accept criteria.