Set as in the collection. Sets are idiomatic, they even have syntax sugar for their construction: #{}.

Seq is extremely leaky, as whether or not a built in function takes a seq or a specific collection is undocumented and arbitrary. Hashmaps for example do not accept seqs when using “into”, it accepts vectors. So my code was littered with (into {} (map vec <thing>)). That’s a leaky abstraction.

You need to hint protocols if you want to go fast.