Looks good! This PR can be a good specimen for the other remaining works. Does "three items" mean OSError, ImportError, BaseException?

Does "three items" mean OSError, ImportError, BaseException?

Yes. I only had to add things to BaseException, since ImportError and OSError inherit from BaseException.

Unfortunately, OSError's and ImportError's reduce act different than BaseException's 😭

• BaseException(https://github.com/python/cpython/blob/3.10/Objects/exceptions.c#L132-L140)
• OSError(https://github.com/python/cpython/blob/3.10/Objects/exceptions.c#L1147-L1194)
• ImportError(https://github.com/python/cpython/blob/3.10/Objects/exceptions.c#L742-L758)

Unfortunately, OSError's and ImportError's reduce act different than BaseException's 😭

I would direct future contributors to look at the extend_exception! macro.

Ok good 😊. Except for the issue about the empty set that I commented on, everything seems fine.