Computing Pi

Throw darts at a square of size 1. Count those that land in an inscribed circle
of radius 1/2 and center (1/2, 1/2).  The area of the circle is pi/4 and the 
areas of the square is one so the ratio of the hits in the circle to the total 
number of trials is pi/4/1 which is pi/4.  Multiply that ratio by 4 to get an 
estimate of pi.  Use random numbers to throw darts. The rand function returns a
random number between 0.0 and 1.0.  Use the time function to time your tests.

If your machine has multiple cores you can use them them to improve the running
time.  Compare
(time (doall (map pi '(200000 200000))))
with
(time (doall (pmap pi '(200000 200000))))