(defn exp
  "exponent of x^n (int n only), with tail recursion and O(logn)"
   [x n]
   (if (< n 0)
     (/ 1 (exp x (- n)))
     (loop [acc 1
            base x
            pow n]
       (if (= pow 0)
         acc
         (if (even? pow)
           (recur acc (* base base) (/ pow 2))
           (recur  (* acc base) base (dec pow)))))))