Comment by steveklabnik

Multiple, in varying degrees of maturity. And I'm also writing one from scratch, don't know if I'll bother to share it with anyone though, I just want to learn more deeply, and implementation is the best way to do that.

I have my eyes on https://github.com/sugyan/atrium as a foundational library in this space, and expect folks to coalesce on it. But we'll see.