I finally find a second–hand Cokin Prism filter, buy it, go try it to a get a feel how it performs—and it damn well falls off the camera onto some cobbles. It’s chipped.
I’m still going to explore it’s capabilities, and, if it’s interesting, I’ll get another. I had one two decades ago and, although I used it occasionally, it did do interesting things. I want to see how it behaves at night.