Guac is a package that provides monadic do-notation, inspired by Haskell, in Python. Monads provide "programmable semicolons" by which the behavior of programs can be changed. A common, useful monad is the list monad, which represents non-deterministic computations. The list monad makes it very easy to write backtracking searches. If you have ever used Python's asyncio package, this may feel familiar. That's because asyncio is actually a monad! Of course, they don't formalize it as such, but it could be implemented as one, and it uses coroutines in the exact same way. Unlike asyncio, which simply continues computation when a result is available, this library makes it possible to repeat computation from arbitrary yields in the coroutine.