proposal to approve test description-logic-502 as an extra credit test

Jim's pointed out that there are very few tests with no systems passing - of
these only one is intrinsically difficult, the other two are recent
additions and either haven't been executed or are just wrong.

I am confident that dl-502 is correct (I ran it through a SAT reasoner
before proposing it) - but it uses aliasing beyond the current capabilities
of the art (unless racer can do it)

I think having it as extra-credit documents a current limitation adequately

Jeremy