Use a flat structure of a set to deal with recursive calls, instead of a...

Use a flat structure of a set to deal with recursive calls, instead of a nested complex, because that is much more efficient and fast in reality.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@37787 20353a03-c40f-0410-a6d1-a30d3c3de9de