code: func addKeyString(xu *xgbutil.XUtil, callback xgbutil.CallbackKey,
  	evtype int, win xproto.Window, keyStr string, grab bool) {
  
  	xu.KeybindsLck.Lock()
  	defer xu.KeybindsLck.Unlock()
  
  	k := xgbutil.KeyString{
  		Str:      keyStr,
  		Callback: callback,
  		Evtype:   evtype,
  		Win:      win,
  		Grab:     grab,
  	}
  	xu.Keystrings = append(xu.Keystrings, k)
  }
docs: // addKeyString adds a new key binding string to XUtil.Keystrings.
  // The invariant is that each key string appears once and only once.