True but the object is for me to understand it noone else....in this case I make an exception

Anyway you don't really need to understand the main stuff, its just weird that the line stays in sync when X<0 but goes out of sync on the other side when it is a mirror of the prgramming for <0.

I can get round this two ways avoiding normal invaders movement. 1. By making the sprites simply go through x>500 back to 0 or just let each sprite see the edge and have then go back and forward overlapping. I still want to do it thw classic way though