fastfib(integer n)
if n < 0 return 0;
else if n = 0 return 0;
else if n = 1 return 1;
else a ← 1; b ← 0;
   for i from 2 to n do
       t ← a; a ← a + b; b ← t;
return a;
end

(defn fast-fib [n]
  (case n
    0 0
    1 1
    (first
      (nth (iterate
             (fn [[a b]]
               (let [t a
                     a (+ a b)
                     b t]
                 [a b]))
             [1 0])
           (- n 1)))))

(defn fast-fib [n]
    (second
      (nth (iterate
             (fn [[a b]] [(+ a b) a])
             [1 0])
           n)))

If n ≥ 2, then at the end of the loop the values of a and b will be xxx and yyy.