defn rot(string, n){
	local i, c, slen;
	slen = len(string);

	i = 0;
	loop 0, slen-1 do {
		c = string[i];
		c = c - 0x61;
		c = c + n;
		c = c % 26;
		c = c + 0x61;
		string[i++] = c;
	}

	return string
}

s="hello"

assert(rot(s, 13) == "uryyb");
assert(s == "uryyb");