Comment by 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)))