gistfile1.txt

    [KSPField(guiName = "Fuel crossfeed",
      isPersistant = true, guiActive = true, guiActiveEditor = true)]
    public bool fuelCrossFeed = false;

    [KSPEvent(guiName = "Toggle crossfeed", name = "ToggleCrossFeed",
      active = true, guiActive = true, guiActiveEditor = true, guiActiveUnfocused = false)]
    public void ToggleCrossFeed()
    {
      part.fuelCrossFeed = fuelCrossFeed = !fuelCrossFeed;
    }