Help... how can I get a clip in to play in reverse?

I have a button that will make a movie clip play going forward… But now I need a the actionscript that will make it play in reverse… can someone help?

In the movie clip :

 onClipEvent (enterFrame) {\r\r     if (_root.i) this.prevFrame () ;\r\r     else play ();\r\r}

Then you can set a button that will do :

 on (rollOver) {_root.i=1;}\r\ron (rollOut) _root.i=0;}

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