[Zope] dtml-tree name

I would like to define the name attribute of the dtml-tree widget with a string returned by a script (called treename) but I'm not sure of the syntax. <dtml-tree expr="<dtml-var treename>" branches="tpValues"> doesn't work nor does <dtml-tree name="<dtml-var treename>" branches="tpValues"> nor <dtml-tree treename branches="tpValues"> Any ideas anyone? TIA Marie Robichon Web Task Force European Synchrotron Radiation Facility Grenoble France