Thanks for the tip. I'd managed to miss the addition of these. I had somehow noticed the addition of the newer string- functions though, and immediately found them a huge improvement over the mishmash of randomly-named crap that was there before, so I expect seq- to be similarly transformative.

I immediately notice there's seq-filter, which can kill off one of my helper routines. And then (now I'm looking...) I've discovered this was always equivalent to cl-remove-if-not. But I never realised, because of the mystery meat naming conventions.