code: func mouseBindGrabs(xu *xgbutil.XUtil, evtype int, win xproto.Window,
  	mods uint16, button xproto.Button) int {
  
  	xu.MousebindsLck.RLock()
  	defer xu.MousebindsLck.RUnlock()
  
  	key := xgbutil.MouseKey{evtype, win, mods, button}
  	return xu.Mousegrabs[key] // returns 0 if key does not exist
  }
docs: // mouseBindGrabs returns the number of grabs on a particular
  // event/window/mods/button combination. Namely, this combination
  // uniquely identifies a grab. If it's repeated, we get BadAccess.