I have a list of input fields and when I tab through them I want to loop back to the first one, but it doesn't seem to work.

Here is my HTML

```
<form id="form">
  <input id="mon" type="text"/> Month<br>
  <input id="day" type="text"/> Day<br>
  <input id="num" type="text"/> Year<br>
  <input id="amt" type="text"/> Amount<br>
</form>

```

and my javascript

```
window.onload=function(){
  $('mon').focus();
  $('amt').onblur=function(){
    //Process the input fields
    $('mon').focus();
  }
}
function $(a){return document.getElementById(a)}

```