Let-Over-Lambda

pick-and-rename.clj

(let [undefined-value (atom :undefined-value)] 
 (defn pick-and-rename [col pick-map]
  (apply assoc
         {}
         (flatten
          (filter (fn [[k v]]
                    (not (= v undefined-value)))
           (map (fn [[k v]]
                  [v (get col k undefined-value)])
                pick-map))))))