Prevent movement off screen

[prabin]

onClipEvent (load) {
speed = 15;
}
onClipEvent (enterFrame) {
if (Key.isDown(Key.LEFT)) {

preXcor=_x;

_x -= speed;
_xscale = -100;

if (_x<0){
_x=preXcor

}
}
if (Key.isDown(Key.RIGHT)) {
preXcor=_x;
_x += speed;
_xscale = 100;
tot=_x+this._width
if (tot>Stage.width){
_x=preXcor;
}


}
}