code: func (p *Point) QuadkeyString(level int) string {
  	s := strconv.FormatInt(p.Quadkey(level), 4)
  
  	// for zero padding
  	zeros := "000000000000000000000000000000"
  	return zeros[:((level+1)-len(s))/2] + s
  }
docs: // QuadkeyString returns the quad key for the given point at the provided level in string form
  // See http://msdn.microsoft.com/en-us/library/bb259689.aspx for more information
  // about this coordinate system.