我这个停止为什么无效？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>定时器</title>
<script type="text/javascript">
var a;
function clock(){
var time=new Date();
var s=time.getHours()+":"+time.getMinutes()+":"+time.getSeconds();
document.getElementById("clock").value=s;
}
function star(){
a=setInterval(clock,1000);
}
function stop(){
clearInterval(a);
}
</script>
</head>
<body>
<form>
<input type="text" size="10" id="clock"/>
<input type="button" value="开始" onclick="star()" />
<input type="button" value="停止" onclicl="stop()" />
</form>
</body>
</html>
