Are there any good sources for exercises (or exercises themselves) for New Foundations, with or without urelements?
I don't know much about New Foundations besides what's listed on its Wikipedia article, which is interesting, but doesn't give a good sense of what it's like to use the theory (even to prove very simple things). The article spends some of its ink talking about whether references to stratification can be confined to purely a syntactic level or not. This is the part that piqued my interest, the stratification rules seem like a special case of type inference ... although it is interesting that the actual types (strata) that were inferred do not play in a role in the set comprehension construction but are simply discarded once we know that the body is typeable.
A lot of posts about NF(U) on this site talk about what it's like to work in the theory compared to ZFC.
Most of the material I've found around the web related to NF(U) talks about open problems in or about NF(U), focuses on comparing NF(U) with other set theories, or talks about consistency results or the provability of the axiom of choice. The closest thing I've found to a collection of exercises is the Metamath Page on the NF collection of proofs ... but picking random theorems in the proof explorer and then trying to prove them myself does not seem like the best way to learn.
