[richhickey]

Because mapcat's signature was not amenable to the additional arity, there's now also flatmap (note you can write the lazy collection version of any transducer fn using sequence as below):

    (defn flatmap
      "maps f over coll and concatenates the results.  Thus function f
      should return a collection.  Returns a transducer when no collection
      is provided."
      ([f]
       (fn [f1]
         (fn
           ([] (f1))
           ([result] (f1 result))
           ([result input]
              (reduce (preserving-reduced f1) result (f input))))))
    
      ([f coll] (sequence (flatmap f) coll)))