Recamán Sequence

recaman.clj

(defn recaman
  ([]
   (recaman 1 1 #{1}))
  ([n prev visited]
   (lazy-seq
     (cons prev
       (let [n    (inc n)
             next (let [lookup (- prev n)]
                    (if (and (pos? lookup) (nil? (visited lookup)))
                      lookup
                      (+ prev n)))]
         (recaman n next (conj visited next)))))))