A question About lineTo()

this.createEmptyMovieClip("a",1);
a.lineStyle(1);
a.moveTo((Stage.width/2),(Stage.height/2));
i=1;
this.onEnterFrame=function()
{
 a.moveTo((Stage.width/2),(Stage.height/2));
 a.lineTo(i,i);
 i=i+10;
}

The above code generates a series of lines…Now what should I do if I want only one line to be drawn at the stage …ie., how to clear the previously drawn line? :ko: