[FMX]Changing a clipEvent into a function

programming flash
1

I have the following clipEvent to control a scrolling menu:

onClipEvent (load) {
 xcenter = 250;
 speed = 1/15;
}
onClipEvent (enterFrame) {
 var distance = _root._xmouse-xcenter;
 _x -= (distance*speed);
 if (_x>0) {
  _x = -500;
 }
 if (_x<-500) {
  _x = 0;
 }
}

How can I make this into a function ?