如何定义一个可以移动的层？

<SCRIPT language=JavaScript>  
    self.onError=null;  
    currentX = currentY = 0;    
    whichIt = null;     
            lastScrollX = 0; lastScrollY = 0;  
    NS = (document.layers) ? 1 : 0;  
    IE = (document.all) ? 1: 0;  
    <!-- STALKER CODE -->  
    function heartBeat() {  
        if(IE) { diffY = document.body.scrollTop; diffX = document.body.scrollLeft; }  
        if(NS) { diffY = self.pageYOffset; diffX = self.pageXOffset; }  
        if(diffY != lastScrollY) {                    percent = .1 * (diffY - lastScrollY);  
                    if(percent > 0) percent = Math.ceil(percent);  
                    else percent = Math.floor(percent);                    if(IE) document.all.Layer1.style.pixelTop += percent;  
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        if(diffX != lastScrollX) {  
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            if(percent > 0) percent = Math.ceil(percent);  
            else percent = Math.floor(percent);  
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            if(NS) document.Layer1.left += percent;  
            lastScrollX = lastScrollX + percent;  
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    }    <!-- /STALKER CODE -->  
    <!-- DRAG DROP CODE -->  
    function checkFocus(x,y) {   
            stalkerx = document.Layer1.pageX;  
            stalkery = document.Layer1.pageY;  
            stalkerwidth = document.Layer1.clip.width;  
            stalkerheight = document.Layer1.clip.height;  
            if( (x > stalkerx && x < (stalkerx+stalkerwidth)) && (y > stalkery && y < (stalkery+stalkerheight))) return true;  
            else return false;  
    }    function grabIt(e) {  
        if(IE) {  
            whichIt = event.srcElement;  
            while (whichIt.id.indexOf("Layer1") == -1) {  
                whichIt = whichIt.parentElement;  
                if (whichIt == null) { return true; }  
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            whichIt.style.pixelLeft = whichIt.offsetLeft;  
            whichIt.style.pixelTop = whichIt.offsetTop;  
            currentX = (event.clientX + document.body.scrollLeft);  
               currentY = (event.clientY + document.body.scrollTop);       
        } else {   
            window.captureEvents(Event.MOUSEMOVE);  
            if(checkFocus (e.pageX,e.pageY)) {   
                    whichIt = document.Layer1;  
                    StalkerTouchedX = e.pageX-document.Layer1.pageX;  
                    StalkerTouchedY = e.pageY-document.Layer1.pageY;  
            }         }  
        return true;  
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    function moveIt(e) {  
        if (whichIt == null) { return false; }  
        if(IE) {  
            newX = (event.clientX + document.body.scrollLeft);  
            newY = (event.clientY + document.body.scrollTop);  
            distanceX = (newX - currentX);    distanceY = (newY - currentY);            currentX = newX;    currentY = newY;  
            whichIt.style.pixelLeft += distanceX;            whichIt.style.pixelTop += distanceY;  
            if(whichIt.style.pixelTop < document.body.scrollTop) whichIt.style.pixelTop = document.body.scrollTop;            if(whichIt.style.pixelLeft < document.body.scrollLeft) whichIt.style.pixelLeft = document.body.scrollLeft;  
            if(whichIt.style.pixelLeft > document.body.offsetWidth - document.body.scrollLeft - whichIt.style.pixelWidth - 20) whichIt.style.pixelLeft = document.body.offsetWidth - whichIt.style.pixelWidth - 20;  
            if(whichIt.style.pixelTop > document.body.offsetHeight + document.body.scrollTop - whichIt.style.pixelHeight - 5) whichIt.style.pixelTop = document.body.offsetHeight + document.body.scrollTop - whichIt.style.pixelHeight - 5;  
            event.returnValue = false;  
        } else {  
            whichIt.moveTo(e.pageX-StalkerTouchedX,e.pageY-StalkerTouchedY);  
            if(whichIt.left < 0+self.pageXOffset) whichIt.left = 0+self.pageXOffset;  
            if(whichIt.top < 0+self.pageYOffset) whichIt.top = 0+self.pageYOffset;  
            if( (whichIt.left + whichIt.clip.width) >= (window.innerWidth+self.pageXOffset-17)) whichIt.left = ((window.innerWidth+self.pageXOffset)-whichIt.clip.width)-17;  
            if( (whichIt.top + whichIt.clip.height) >= (window.innerHeight+self.pageYOffset-17)) whichIt.top = ((window.innerHeight+self.pageYOffset)-whichIt.clip.height)-17;  
            return false;  
        }  
        return false;  
    }  
    function dropIt() {  
        whichIt = null;  
        if(NS) window.releaseEvents (Event.MOUSEMOVE);  
        return true;  
    }  
    <!-- DRAG DROP CODE -->  
    if(NS) {  
        window.captureEvents(Event.MOUSEUP|Event.MOUSEDOWN);  
        window.onmousedown = grabIt;  
         window.onmousemove = moveIt;  
        window.onmouseup = dropIt;  
    }  
    if(IE) {  
        document.onmousedown = grabIt;  
         document.onmousemove = moveIt;  
        document.onmouseup = dropIt;  
    }  
    if(NS || IE) action = window.setInterval("heartBeat()",1);
</SCRIPT>