[Zope] newbie-prob: dtml-in ... sort=myVar

[markus]

hi,
a page is called via links like

1.) myPage.html?sortkey=Date
2.) myPage.html?sortkey=Name

the value of sortkey is display properly with 
<dtml-var sortkey>

i like to use the value of sortkey within the <dtml-in> -tag, like

<dtml-in expr="objectItems(['DTML Document'])" sort=_['sortkey']>

but that is not working...

the tags
<dtml-in expr="objectItems(['DTML Document'])" sort=Date>
<dtml-in expr="objectItems(['DTML Document'])" sort=Name>
are doing fine, but that's not a solution for my prop.

please help

thanks in advance
cheers, Markus