Stuff from LOL translated into Clojure

gistfile1.clj

; listing 1.4: group
(defn group [coll n]
  (if (zero? n)
    (throw (Exception. "zero length"))
    (partition-all n coll)))

; listing 1.5: flatten
; built-in

; listing 1.6: fact-and-choose
(defn fact [x]
  (loop [n x total 1]
    (if (zero? n)
      total
      (recur (dec n) (* total n)))))

(defn choose [n r]
  (/ (fact n)
     (fact (- n r))
     (fact r)))
(def ncr choose)