As discussed in this thread, I am submitting a more performant version of flatten for review. It has the same semantics as the current core/flatten. I have also updated the doc string to say that "(flatten nil) returns the empty list", because that's what the current version of core/flatten does as well.
I haven't mailed in a CA yet, but I will tomorrow morning.
Edit: Of course I'd mess my first ticket up . I am not immediately seeing an option to edit this to add the "patch" tag or add the "ready to test" action. Sorry folks