Michael Barr wrote in part:
Another affectation is using "topoi" as the plural of topos. If you insist on that, you should use the genetive of "of topos" and the accusative when it is the direct object--not to mention the vocative when addressing a topos.
That's not really fair; there's a long history in English, when adopting a foreign noun, of adopting the foreign plural as well (then switching to an "-s" plural when the noun becomes less foreign, or occasionally using the plural form only as a collective noun). However, this practice uses only one case, usually nominative. So by saying "topoi", one pretends that "topos" is a real Greek word (in this sense) and that it's still a foreign word with a foreign plural. This pretence is an affectation, certainly, but it is complete in itself. --Toby