ActionScript 2.0 :: SetMask - Make Rectangle Clip Visible Through Drawn Line?

I've got a movieclip of a rectangle in my library which I place on stage to attachMovie. Then I create an empty movieclip which I want to use as a mask on that rectangle clip through setMask. Users kan draw into that empty movieclip. My intention is that only those areas of the rectangle clip become visible where the user is drawing a line. So I used this:

Actionscript Code:
_root.attachMovie("clip", "clip", 0);
_root.createEmptyMovieClip("line", 1);
clip.setMask(line);_root.onMouseDown = function() {
line.moveTo(_xmouse, _ymouse);
line.lineStyle(10, 0x00FF00, 100);
this.onMouseMove = function() {
line.lineTo(_xmouse, _ymouse);
updateAfterEvent();};};
_root.onMouseUp = function() {
this.onMouseMove = null;};

But that doesn't work: both the line drawn and the rectangle clip become invisible. I know it works because without the setMask part the line is drawn. The other way around does work (line.setMask(clip)): the rectangle clip is now masking the drawn line so only the line becomes visible within the rectangle region. But I want it the other way around: the rectangle should become visible through the lines that are drawn!