Destructuring is also built in. It's not clear to me if there's a destructuring use case for (bar 12) in Arc, but I could easily see using it the way Erlang uses its pattern matching, so that (def myfn (foo (bar 12 _)) (do things)) would mean that myfn was only called if the cadr of the second argument was 12. Since '_' in this case means "don't care", I'm not sure there'd be any conflict with '_' as already used in Arc.
