如果调整select中option的值

<script>
function move(bool)
{
i=bool=="up"?-1:1;
myvalue=document.all.mysel.options[document.all.mysel.selectedIndex+i].value;
mytext=document.all.mysel.options[document.all.mysel.selectedIndex+i].text;
document.all.mysel.options[document.all.mysel.selectedIndex+i].value=document.all.mysel.options[document.all.mysel.selectedIndex].value;
document.all.mysel.options[document.all.mysel.selectedIndex+i].text=document.all.mysel.options[document.all.mysel.selectedIndex].text;
document.all.mysel.options[document.all.mysel.selectedIndex].value=myvalue;
document.all.mysel.options[document.all.mysel.selectedIndex].text=mytext;
}
function set()
{
if(document.all.mysel.selectedIndex==0)
document.all.up.disabled=true;
else
document.all.up.disabled=false;
if(document.all.mysel.selectedIndex==document.all.mysel.options.length-1)
document.all.down.disabled=true;
else
document.all.down.disabled=false;
}
</script>
<body onload=set()>
<select id=mysel size=4 onchange=set()>
<option value=1 selected>1
<option value=2>2
<option value=3>3
<option value=4>4
</select>
<input type=button value=up id=up onclick=move("up")>
<input type=button value=down id=down onclick=move("down")>