[Zope] Variable substitution in an object name

I can substitute a property name for the base of an object call with: <dtml-var expr="_[wrap].index_html(_.None,_)"> where wrap is a property that refers the name of an object But I have not been able to figure out how to use a variable to substitute the method (in this case index_html but could be index_xml or index_wap) If I use the above methodology - I get looping login boxes where it won't authenticate and when I hit cancel I get a traceback in file DT_Util.py line 142, in careful_getattr. Is there a way to do this?? Thanks Lou