code: func PropValNum64(reply *xproto.GetPropertyReply, err error) (int64, error) {
  	if err != nil {
  		return 0, err
  	}
  	if reply.Format != 32 {
  		return 0, fmt.Errorf("PropValNum: Expected format 32 but got %d",
  			reply.Format)
  	}
  	return int64(xgb.Get32(reply.Value)), nil
  }
docs: // PropValNum64 transforms a GetPropertyReply struct into a 64 bit
  // integer. Useful when the property value is a single integer.