r and i are natural numbers, not both zero,
and m is a positive integer.
Major Section: PROGRAMMING
(mod-expt r i m) is the result of raising the number r to the
integer power i and then taking the residue mod m. That is,
(mod-expt r i m) is equal to (mod (expt r i) m).
The guard for (mod-expt r i m) is that r is a rational number
and i is an integer; if r is 0 then i is nonnegative; and
m is a non-zero rational number.
In some implementations (GCL Version 2.7.0 as of this writing), this function
is highly optimized when r and i are natural numbers, not both zero,
and m is a positive integer.