Thanks for the thorough rundown, Nathaniel. I started to get an idea of the
required shape only by looking at CPython code like you suggest. I wanted
to create an awaitable compatible with asyncio and trio that could be
awaited more than once unlike a coroutine, and not runner-specific like a
"Awaitable" is a language-level concept. To actually use awaitables,
you also need a coroutine runner library, and each library defines
additional restrictions on the awaitables it works with. So e.g. when
using asyncio as your coroutine runner, asyncio expects your
awaitables to follow particular
