[Zope] Customising the calling of the _add method for a product

[icottee@bluefountain.com]

I have a TimeSheet product which is currently utilising the suggestions from one
of the howtos on zope.org, to make the handling of errors a little smarter.
Subsequently my addForm code checks to see if any errors were detected and, if
not, tries to go to the usual _add method. But this fails. The code below shows
what I currently have. If I can just call this _add method I am finished.

 <dtml-if "I_Error==1">
    <dtml-var TimeSheet_addForm_include>
 <dtml-else>
       <dtml-call "TimeSheet_add(_,_.None)">
 </dtml-if>

As always, any help greatfully received

Ian