[AS2] Is confused about a path to a MC

[tongxn]

onClipEvent (load){i3 = _parent.getNextHighestDepth();}
onClipEvent (mouseMove) {
    i3 ++ 
//weird, because trace(i3) actually returns a increasing number.
    attachMovie("snow","snow"+i3,i3);
    with ("_level"+i3["snow"+i3]) { //working
        _x = 0+40-80*Math.random();
        _y = 0+45-90*Math.random();
        _rotation = 60*Math.random();
        _xscale = _yscale=30*Math.random();
        _alpha = 100;
    }
    trace ("_level"+i3["snow"+i3].getDepth()); //output: _levelundefined
    this.onEnterFrame = function() { //working
        with (this) {
            _x += _xscale/13;
            _y -= _yscale/30;
            _alpha -= 2;
            if (_x>896 || _y>672 || _alpha<0) {
                this.removeMovieClip();
            }
        }
    };
    updateAfterEvent();
} 

I made it work by accident, but now the ["snow"+i3] is on _levelundefined according to the trace().
but didn't I attach the clip to the depth i3?