Bob Engelhardt writes:

I could be wrong, of course - it has been 17 years since I retired.
If so, I'd be very interested in seeing how software is modeled. Just
hearing what the parameters are would be interesting. Maybe I should
find a software NG to ask.

Not all models are purely numbers/math based.

Some work is being done in ontologies and the Semantic Web. I just
attended an interesting talk on the subject. One of the problems is
that rules for an expert systems are based on the model of the system
underneath.

There can be a large gap there in these two models. 10 years ago, I
was a domain expert for a new expert system. I knew when a system was
behaving properly, and how to diagnose a system that didn't have
maximum performance. The challenge was how to owrute rules that
expressed my knowledge so automated systems can duplicate my
skill. Expressing rules that a human "knows" in a form a computer can
use is a challenge.

So now - new languages are being created to describe the system
underneath. These languages are fully integrated into the Integrated
Development Environment (IDE). Try to describe a rule for a system,
and the editor lets you know there's a semantic error, forcing you to
correct the rule as you type it in. So you can either define a new
rule, or extend the model underneath, using the same IDE.