Greenstack is a fork of greenlet, which is a spin-off of Stackless, a version of CPython that supports micro-threads called "tasklets". Tasklets run pseudo-concurrently (typically in a single or a few OS-level threads) and are synchronized with data exchanges on "channels".
A "greenlet", on the other hand, is a still more primitive notion of micro-thread with no implicit scheduling; coroutines, in other words. This is useful when you want to control exactly when your code runs. You can build custom scheduled micro-threads on top of greenlet; however, it seems that greenlets are useful on their own as a way to make advanced control flow structures. For example, we can recreate generators; the difference with Python's own generators is that our generators can call nested functions and the nested functions can yield values too. Additionally, you don't need a "yield" keyword. See the example in tests/test_generator.py.
Greenlets are provided as a C extension module for the regular unmodified interpreter.
Greenlets are coroutines for in-process concurrent programming.
Greenstack is on pypi, so you can install it thus:
pip install greenstack
If you're using a library such as Gevent or Eventlet that directly uses greenlet,
you can install greenstack-greenlet. This installs a
`greenlet/__init__.py file`__ that makes Greenstack look like Greenlet.