code: func (w *Window) Listen(evMasks ...int) error {
  	evMask := 0
  	for _, mask := range evMasks {
  		evMask |= mask
  	}
  	return xproto.ChangeWindowAttributesChecked(w.X.Conn(), w.Id,
  		xproto.CwEventMask, []uint32{uint32(evMask)}).Check()
  }
docs: // Listen will tell X to report events corresponding to the event masks
  // provided for the given window. If a call to Listen is omitted, you will
  // not receive the events you desire.
  // Event masks are constants declare in the xgb/xproto package starting with the
  // EventMask prefix.