Clojure code to estimate pi

pi.clj

; for some reason it's not finding clojure.contrib.math
;(ns pi-estimator
;  (:require clojure.contrib.math))

; this didn't work either:
;(ns pi-estimator
;  (:use [clojure.contrib.math :only [abs]]))

(defn abs [x]
  (if (>= x 0)
    x
    (* x -1)))

(defn estimate-pi [tol]
  (loop [est 0.0 err 4.0 mul 1 denom 1]
    (if (< err tol)
      est
      (let [new-est (+ est (/ (* 4.0 mul) denom))]
        (recur new-est (abs (- new-est est)) (* -1 mul) (+ 2 denom))))))

(println "Pi accurate to five places:" (estimate-pi 0.00001))