(you'll note that very few changes are required to

@rule

bodies, except for converting a final

yield

to

return

to conform with mypy's expectation for generator methods. this isn't a breaking change, and it allows gradual migration, although i assume we'd want to convert `@rule`s pretty quickly to the type-safe syntax as we did with the initial mypy

@rule

typing PR. no rush on my end, though)