code: func (w *Window) CreateChecked(parent xproto.Window, x, y, width, height,
  	valueMask int, valueList ...uint32) error {
  
  	s := w.X.Screen()
  	return xproto.CreateWindowChecked(w.X.Conn(),
  		s.RootDepth, w.Id, parent,
  		int16(x), int16(y), uint16(width), uint16(height), 0,
  		xproto.WindowClassInputOutput, s.RootVisual,
  		uint32(valueMask), valueList).Check()
  }
docs: // CreateChecked issues a CreateWindow checked request for Window.
  // A checked request is a synchronous request. Meaning that if the request
  // fails, you can get the error returned to you. However, it also forced your
  // program to block for a round trip to the X server, so it is slower.
  // See the docs for Create for more info.