strange Element.implement() problem in IE6

[Oren]

Hey guys - I'm having this problem only in IE6... it's saying for
"el.setCleanse()", "This method not supported by Object", and when I
do a for in alert loop to look at all the properties of el, it's only
native object stuff, nothing from mootools.  I've tried casting with $
(), doing $(el.id), no luck.  Maybe something to do with
Element.implement?  I see no reason why this code shouldn't work... I
do the same thing in other sites all over the place =(

<code>
  Element.implement({
        setCleanse: function(ceiling,zero)
{ this.addEvent('keyup',function(){ .. } })
  });

  $('cart').getElements('input[id^=qty]').each(function(el)
{el.setCleanse(0,true)});
</code>