연습문제 1.16

function fast_expt(b, n) {
    return fast_expt_iter(b, n, 1);
}

function fast_expt_iter(b, counter, product) {
    return counter === 0
        ? product
        : is_even(counter)
        ? fast_expt_iter(b * b, counter/2, product)
        : fast_expt_iter(b, counter-1, b* product);
}

function sqaure(n) {
    return n * n;
}

function is_even(n) {
    return n % 2 === 0;
}