Maps, keywords, and symbols when used as operators allow optional second arguments for 'default-not-found' values is if to 'get'.
({:a 1} :b 'b) => b
However sets don't support this behavior (though they do with 'get') and vectors don't allow the optional default-not-found in their pseudo 'nth' semantics.
user=> (#{:a :b} :b 'notfound)
ArityException Wrong number of args (2) passed to: PersistentHashSet clojure.lang.AFn.throwArity (AFn.java:437)