Assertions should be more debugger-oriented

Asserts play different roles:

1. Invariant assurance when entering/exiting an operation.

2. Argument checking. Enforcing requirements as to how an API is to be used.

3. Documenting the state of an operation as it proceeds. This is a kind of logging.

4. Acting like a kind of insiders (pre-)unit-testing. Ensuring certain assumptions are not violated as a function is put together, or as it evolves in the future.

I consider debugging, manual (initial) testing, automated testing, asserts, logging, long-term monitoring, and even commenting as somewhat related. But I don't know the best way to unify these things or even if that's desirable.

You can type `f 4` to jump directly to the function running before entering the assertion machinery.