code: func WmWindowOpacitySet(xu *xgbutil.XUtil, win xproto.Window,
  	opacity float64) error {
  
  	return xprop.ChangeProp32(xu, win, "_NET_WM_WINDOW_OPACITY", "CARDINAL",
  		uint(opacity*0xffffffff))
  }
docs: // _NET_WM_WINDOW_OPACITY set