code: func NewBoundFromMapTile(x, y, z uint64) *Bound {
  	maxIndex := uint64(1) << z
  	if x < 0 || y < 0 || x >= maxIndex || y >= maxIndex {
  		panic("tile index out of range")
  	}
  
  	shift := 31 - z
  	if z > 31 {
  		shift = 0
  	}
  
  	lng1, lat1 := scalarMercatorInverse(x<<shift, y<<shift, 31)
  	lng2, lat2 := scalarMercatorInverse((x+1)<<shift, (y+1)<<shift, 31)
  
  	return &Bound{
  		sw: &Point{math.Min(lng1, lng2), math.Min(lat1, lat2)},
  		ne: &Point{math.Max(lng1, lng2), math.Max(lat1, lat2)},
  	}
  }
docs: // NewBoundFromMapTile creates a bound given an online map tile index.
  // Panics if x or y is out of range for zoom level.