Add comments.
Add safe fallback for a strange case: we get a variable value combination which is not claimed by any worklist. It can come from a query optimisation bug in python code, but possibly other causes 
must be identified.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@16173 20353a03-c40f-0410-a6d1-a30d3c3de9de