As that'll work regardless of where you are in the filesystem (location-dependent commands are a typical shell gaffe). What I'd really like, however, is to be able to go into a “solve” mode where ...

If it's not, it tells you what's left over. For example, if you divide 3 by 2, the MOD function will return 1. In this guide, I'll show you how you can use it to solve real-world problems.