code: func LargestOverlap(needle Rect, haystack []Rect) int {
  	biggestArea := 0
  	reti := -1
  
  	var area int
  	for i, possible := range haystack {
  		area = IntersectArea(needle, possible)
  		if area > biggestArea {
  			biggestArea = area
  			reti = i
  		}
  	}
  	return reti
  }
docs: // LargestOverlap returns the index of the rectangle in 'haystack' that has the
  // largest overlap with the rectangle 'needle'.
  // This is commonly used to find which monitor a window should belong on.
  // (Since it can technically be partially displayed on more than one monitor
  // at a time.)
  // Be careful, the return value can be -1 if there is no overlap.