如何实现在下拉菜单里输入文字？

<script>
function pp(){
se.options[2]=new Option(ok=(se.options[2])?se.options[2].innerText+String.fromCharCode
(event.keyCode):String.fromCharCode(event.keyCode),"client")
se.selectedIndex=2;
}

function edit(){
if(se.options[2]){
if(event.keyCode==8){
var str=se.options[2].innerText;
var len=str.length;
se.options[2].innerText=str.substring(0,len-1);
if(se.options[2].innerText=="")se.remove(2);
}
if(event.keyCode==13)return false;
if(event.keyCode==32){
se.options[2].innerText+=" ";
}
}
}
</script>
<select id=se onkeypress=pp() onkeyup="edit()">
<option>山水盈盈之输入文字
</select>