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() ">

<option>山水盈盈之输入文字

</select>