distinctp.clj

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