Hm, how do you determine whether something is a collection operation versus a seq operation? Sure, the docstring of `shuffle` says "Return a random permutation of coll" and the argument is also called `coll` but the same is true for say `filter`. Also, calling `shuffle` with a non-nil (lazy) seq works fine, too.
Aha, now I see that the argument for `shuffle` is tagged with `java.util.Collection` but that's more like an implementation detail, no? Seems inconsistent to me to not also accept `nil` here due to the above.