More transitive-property work. - Add some more tests that break the current SQL backend implementation for transitive properties. One of them looks like a Postgres bug to me -- I'm getting: """ProgrammingError: ERROR: relation "_user2" does not exist""" where _user2 is an alias for _user in the from clause (but _user2 is used in a left outer join). The other SQL backends are fine with the generated SQL. Hmm. One of the reasons for packing sort and search attributes into the same data structure was the optimized SQL: not creating a left outer join for attributes that are already in a normal join (from/where clause). I'll check if I can instead leave the attribute from the from-clause instead (or if I'm then getting errors from the other backends, Postgres seems fine with that).