[Greg Wilson]
... Unfortunately, if values are required to be immutable, then sets of sets aren't possible... :-(
Sure they are. I wrote about how before, and Moshe put up a simple implementation as a SourceForge patch. Not bulletproof, though: "consentng adults". No matter *what* you implement, I'll find *some* way to trick it into believing my sets are immutable <wink>, so don't worry about that. Bulletproof is very hard, and is a minority distraction at best. IIRC, SETL had "by value" semantics when inserting a set into another set as an element, and had some exceedingly hairy copy-on-write scheme under the covers to make that bearably quick. That may be wrong, though. Herman Venter's Slim (Sets, Lists and Maps) language does work that way (Guido, Herman was a friend of the departed Stoffel Erasmus, who you may recall fondly from Python's very early days -- if *that* doesn't make sets attractive to you, nothing will <wink>). Ah! Meant to post this before: http://birch.eecs.lehigh.edu/~bacon/setlprog.ps.gz That's a readable and very good intro to SETL Classic. People pondering computerized sets should at least catch up with what was common knowledge 30 years ago <wink>.