produces some Y and then we apply a G of that of that we should get back to X okay and this is a constraint that people have used a lot especially in tracking in computer vision you track forwards you get somewhere you don't know where you are then you track backwards in time along the video and the idea is that you you should end up where you started with and if you don't then something is wrong okay but we can use this as a constraint to again for optimization so for example let's say that we want to do this kind of a pics