# modexp ( base, exp, n ) = base**exp mod n
sub modexp {
    my ($base, $exp, $n) = @_;
    die "Negative exponent" if $exp<0;
    my $res = 1;
    while ($exp>0) {
	$res = ( $res * $base ) % $n;
	$exp--;
    }
    return $res;
}