[Versa] Versa issue: transitive closure function proposal

[Uche Ogbuji]

Timor reported a need an submitted a patch for a transitive-closure
function.  His docstring is as follows:

"""
Apply an expression transitively, starting with the given set of values
(resources), making sure to avoid circularity. The expression could
return nested lists/sets, in which case, every object in the topmost
list is searched for a resource terminating the leftmost path in the
tree and the found objects are used for the next transitive lookup.
"""

He explained his particular need a bit more here:

http://lists.fourthought.com/pipermail/4suite/2005-March/013413.html

I tend to think we should have a transitive closure function, but I'm a
bit unsure of Timor's choices of how to deal with complex subquery
results.

Timor, any help you can provide in outlining more general use cases
would be excellent.

Thanks.

-- 
Uche Ogbuji                               Fourthought, Inc.
http://uche.ogbuji.net                    http://fourthought.com
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