这段拖拽代码，有个小问题，不吝赐教

<!--接上面--> 
<body>
<INPUT TYPE="button" value="添加DIV" onclick="add_div();">
<div class=content>
        <div class=left id=dom0>
                <div class=mo id=m0>
                        <h1>dom0</h1>
                        <div class="nr"></div>
                </div>
                <div class=mo id=m1>
                        <h1>dom1</h1><div class="nr"></div>
                </div>
                <div class=mo id=m2><h1>dom2</h1><div class="nr"></div></div>
                <div class=mo id=m3><h1>dom3</h1><div class="nr"></div></div>
        </div>
        <div class=center id=dom1>
                <div class=mo id=m4><h1>dom4</h1><div class="nr"></div></div>
                <div class=mo id=m5><h1>dom5</h1><div class="nr"></div></div>
                <div class=mo id=m6><h1>dom6</h1><div class="nr"></div></div>
                <div class=mo id=m7><h1>dom7</h1><div class="nr"></div></div>
        </div>
        <div class=right id=dom2>
                <div class=mo id=m8><h1>dom8</h1><div class="nr"></div></div>
                <div class=mo id=m9><h1>dom9</h1><div class="nr"></div></div>
                <div class=mo id=m10><h1>dom10</h1><div class="nr"></div></div>
                <div class=mo id=m11><h1>dom11</h1><div class="nr"></div></div>
        </div>
</div></body>
</html>