formfragment?

Hi guys,

I'm trying to get the form fragment mixin working, but the fragment is not
hidden when the page loads, nor does anything change as i check or uncheck
the checkbox. here is the relevant part of the tml:

<tr>
<td class="name">Trifurcation </td>
<td><t:checkbox t:mixins="triggerfragment"
fragment="trifurcationdetails" t:id="trifurcation"/></td>
</tr>

<t:formfragment t:id="trifurcationdetails"
visible="trifurcation">
<tr>
<td class="name">Number of segments of disease </td>
<td><t:select t:type="select"
t:id="trifurcationNumberOfSegments" t:model="literal:1,2,3,4,5,6,7,8"
t:blankOption="ALWAYS"/></td>
</tr>
</t:formfragment>

any ideas?