fibonacci identity

fib.hs

choose _ 0 = 1
choose 0 _ = 0
choose n k = choose (n-1) (k-1) *	n `div` k

pow n 0 = 1
pow n k	= n * pow n (k-1)

fib n = theSum `div` pow2
  where theSum = sum [ (choose n (2*k+1)) * (pow 5 k) |
                        k <- [0 .. div (n-1) 2] ]
        pow2 = pow 2 (n-1)