on AS3 - Erase a Bezier curve.

Thank you. This is a portion of my code. If I remove the clear() line, the screen fills up with Beziers. If I leave the clear() line, the page remains blank. It looks like once the clear() line is there I can not draw on the sprite any more.

var yy:int= 10;

var ctrlPt:int = 0;

addEventListener(Event.ENTER_FRAME, onEnterFrame);

function onEnterFrame(event:Event):void

{

yy++

ctrlPt = yy + 80;

sp.graphics.clear();

sp.graphics.moveTo(200,yy);

sp.graphics.curveTo(300,ctrlPt,400,yy);

addChild(sp);

}