@MartinSleziak I mean "hackable" in the same way that a text editor like vim is "hackable" --- it is easy to modify and change the code to suit ones needs. For instance, I might want to be able to embed it, or make a web interface to it, or something like that.

Of course, this wants something that is both open-source and somewhat modular, which is a bit complicated.

I've been paying attention to Approach0. I haven't actually looked through its codebase yet, but it's on my long list of things to think about.

It's certainly permissable enough (in the sense that it's licensed under the MIT license, so one can change it to one's whim).