[Zope] sort with external comparison function

I have a problem with sorting in a <dtml-in> which may be a bug or just a conctptual error. Suppose foo is a simple sequence and myCompare is a two-parameter Python script returning -1, 0, 1. The the dtml snippet <dtml-in foo sort="sequence-item/myCompoare"> <dtml-var sequence-item> </dtml-in> should render the sequence in my sort order but does not. It does call the myCompare routine (a Python Script in the Zope space). The problem seems to be with the parameterization. The version of myCompare I am trying to use generates an error because the parametrs seem not to be set. If I change myCompare to be def myCompare( a,b): return cmp( a, b ) it should be the equivalent of the default sort--but it's not.