How to make a moving wall?

How to make wall moving with player forward, but when player goes back wall doesnt move. It's like in Mario land where you can only go forward.

I've made a little example ->

http://dl.dropbox.com/u/26932498/scroll.cap

Put the 90 at "scrollxleft + 90" to a little lower number, to put the "scroll-wall" exactly on the left screen, so the player doesn't see it.

edit: reuploaded the .cap, fixed a small bug^^ You can also check the option "set invisible on start" of the wall. And the condition "player is walking" can be deleted. Dunno why I added it.

thanks!