Yup, makes sense (well, paragraph three onward -- I don't know enough yet to understand stuff like "dotted type-variable", but I think the general way this is working is clear :).

So would this definition be equivalent and work?

  (ann clojure.core/map
    (All [c b ...]
      (Fn [[b ... b -> c] 
           (NonEmptySeqable b) ... b 
           -> (NonEmptyLazySeq c)]
          [[b ... b -> c] 
           (U nil (Seqable b)) ... b
           -> (LazySeq c)])))

| The b's "match up" pairwise, but they're quite different than what you might expect.

Makes sense, but slightly confusing at first, 'cuz the b before the ... is a different thing than the b after.

Thanks for the response, and typed clojure. I think this is going to be the thing that finally gets me to start playing with clojure. :)