[ezy]

As someone who is unfamiliar with clojure -- is there any particular reason a function which doesn't actually work with sets allows non-sets to be passed to it in the first place? Or this a deficiency in the type-checking? I mean, truly, this kind of thing seems like a solved problem...

[spicy_meatball]

This is a tradeoff made with dynamically-typed languages. Open up your dev console and try some JS for other examples: `[] + []` or `[] + {}` or `{} + []` or `{} + {}`.

There are several ways to solve it all with tradeoffs. Types can solve it with tradeoffs. Checking the types in the function can solve it with tradeoffs. Stating in the docs what arguments may be passed is another way to solve it.