Via issue 28410, Serhiy is adding a private "_PyErr_FormatFromCause" helper API to make explicit C level exception chaining easier within the implementation of CPython (this helps resolve issue 28214, an exception reporting problem in the new PEP 487 __set_name__ feature).

It may be sufficient to make that API public, and use that as the home of documentation for some of the subtleties discussed here.