Thanks both, I now feel more qualified, so to speak. It’s a little unintuitive to me that one would use the original namespace to define a new method, as in:
NiceStuff.nice(::Cat) = "nice 😸"
but now I know that’s the syntax to use.
The alternative also surprised me,
nice(::Cat) = "nice 😸"
which is what I’d naively started with, but I was only using .NiceStuff in my module, where in fact I need to also (/alternatively) import .NiceStuff: nice to make it available unqualified.