A time ago I had a project where we had an intense tree-like data structure where at any given level there may be a finite async-stream. Basically recursively needed to await streams until they finished.

I was using for RxDB based on RxJS, so after a couple nights of banging my head I went into the RxJS chat and asked for help. One of the maintainers of the library who worked at MS said it was a perfect problem and I think I nerd-baited him into helping out...

... we ended up spending like a week on it going back and forth, and neither of us could figure it out. The code we ended up with was frightening.