*fast*) implementations.

Here is an example by Jochen Schmidt of how EXPT-MOD can be implemented in portable
Common Lisp:

(defun expt-mod (n exponent modulus) "As (mod (expt n exponent) modulus), but more efficient." (declare (optimize (speed 3) (safety 0) (space 0) (debug 0))) (loop with result = 1 for i of-type fixnum from 0 below (integer-length exponent) for sqr = n then (mod (* sqr sqr) modulus) when (logbitp i exponent) do (setf result (mod (* result sqr) modulus)) finally (return result)))

*Q: Why standardize an interface (well, a function name)? Lisp implementations that want to provide an accelerated (expt (mod ...)) can do that with a compiler macro, right?*
*A: Yes, but if you rely on that compiler macro as programmer you will get horrible performance, if your implementation does not have an optimized algorithm for (expt (mod ...))*