Maps, keywords, and symbols when used as operators allow optional second arguments for 'default-not-found' values is if to 'get'.
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.
I fixed the problem with Dirk at the Amsterdam Clojurians Hackathon.
Guess you can add a couple of unit tests as well.
I have added some tests for the changes made earlier.
Is there something that is needed for these patches to be applied? I run into this semi-regularly.
Sorry, has just never been looked at afaik. Votes help raise visibility.