"Screen" algo in mt_lutxy

[Dogway]

I borrowed the polish syntax from another function, but couldnt make the screen blend mode to perform correctly. It is most noticeable when black over black, where luma should be 16 is 31. Im using the next code:

mt_lutxy(a,b,"256 256 x - 256 y - * 256 / - " ,Y=3, U=2, V=2)

I also found in another web the infix syntax, I tried using it like so:

mt_lutxy(a,b,mt_infix("255 - (255-U)*(255-L)/255 ***"),Y=3, U=2, V=2)

but everything renders black.