てきとう

a.js

const succ = (a) => a + 1;
const pred = (a) => a - 1;

const add = (a, b) => {
	if (b === 0) { return a; }
	else { return add(succ(a), pred(b)); }
}

const mul = (a, b) => {
	if (b === 0) { return 0; }
	else { return add(a, mul(a, pred(b))); }
}

const pow = (a, b) => {
	if (b === 0) { return 1; }
	else { return mul(a, pow(a, pred(b))); }
}

console.trace(pow(21, 3));

a.rb

def succ (a)
	return a + 1
end

def pred (a)
	return a - 1
end

def add(a, b)
	if (b == 0) then
		return a
	end
	return add(succ(a), pred(b))
end

def mul(a, b)
	if (b == 0) then
		return 0;
	end
	return add(a, mul(a, pred(b)))
end

def pow(a, b)
	if (b == 0) then
		return 1;
	end
	return mul(a, pow(a, pred(b)))
end

puts pow(21, 3)