Which packages do you know of, that have the following properties:
- using custom notation (for input AND output would be preferable)
- well coded & designed (from your subjective perspective!)
Mathematica Stack Exchange is a question and answer site for users of Wolfram Mathematica. It only takes a minute to sign up.Sign up to join this community
I'm not going into the well-coded part of your question (as this is rather subjective), but a package that I've (cursorily) examined and which looks nice is this quantum notation package, which has lots of custom notation and corresponding palettes.
Besides the quantum package already mentioned by @Sjoerd, the package with the most customized notation that I know of is the THEOREMA package. You can freely use the package and admire the complex logicographics notation created, but the code is not available for inspection.
Finally, the OP leaves me No-Escape (pun intended) but to mention my WildCats category theory package which is perhaps a unique example of 3rd party package using the standard Notation package together with some hand-made (MakeExpression, etc.) custom notation. You can inspect my code.