* Finish off re-implementing all mtl classes.
* Implement being able to change out the underlying stack from a
transformer (e.g. `StateT s m a -> StateT s n a`).
* Clean up API.
- Determine what needs to be exported
- Should variadic-functions be exported from Levels.hs?
- Change Constraint synonyms to type-classes for better error
reporting?
- Should VArgs be able to have different constraints for
lifting/lowering?
* Documentation
- Document modules
- Add extra documentation modules:
+ How to use the library
+ How to add extra base monads
+ How to add extra levels
+ How to add a new transformer class
+ Dealing with variadic functions
* Test suite
* Benchmarking and performance enhancements