distinct_key.clj

(defn distinct-key
  "Returns a lazy sequence of the elements of coll with (k element) duplicates removed"
  [k coll]
    (let [step (fn step [xs seen]
                 (lazy-seq
                   ((fn [[f :as xs] seen]
                      (when-let [s (seq xs)]
                        (let [key (k f)]
                          (if (contains? seen key) 
                            (recur (rest s) seen)
                            (cons f (step (rest s) (conj seen key)))))))
                    xs seen)))]
      (step coll #{})))