> Example of iterative vs tail-recursive code.  The former works in
> GCL 2.4.0, the latter slags with a invocation history stack overflow.
>
> ;;; do modular exponentiation - get (B**E) mod M.
>
> ;;; general driver
> (defun expmod (base exponent modulus)
>   (expmod-iter base exponent modulus))
>
> ;;; iterative algorithm (aye, matey, this be bletcherous)
> (defun expmod-iter (x b n)
>   (let ((z 1)
> (b b)
> (x x))
>     (loop
>      (when (= b 0) (return z))
>      (loop
>       (when (not (= (rem b 2) 0)) (return))
>       (setf b (/ b 2))
>       (setf x (rem (* x x) n)))
>      (setf b (- b 1))
>      (setf z (rem (* z x) n)))))
>
>
> ;;; tail-recursive algorithm from SICP
> (defun expmod-rec (base exponent modulus)
>   (cond ((= exponent 0) 1)
> ((evenp exponent)
> (rem (square (expmod base (floor (/ exponent 2)) modulus))
>       modulus))
>         (t
> (rem (* base (expmod base (- exponent 1) modulus))
>       modulus))))
>
> (defun square(x) (* x x))
>
>