umm ok

I didn't need an expression to do this. Instead, I just

* keyframed the tx and ty curves for the first 40 frames.

* Opened the graph editor

* Picked the ty curve and selected all of its keys by dragging

* chose Curves > Post Infinity > Cycle

* Picked the tx curve and selected its keys

* chose Curves > Post Infinity > Cycle with Offset

No expressions at all, and as far as I can tell it does exactly what you want.

-- Mark