Clojure Quick Sort with random pivot

quicksort.clj

(defn quick-sort
  "Sort sequence using simple recursion based on splitting by randomly chosen pivot"
  [xs]
  (if (empty? (rest xs))
    xs
    (let [pivot-index
          (rand-int (count xs))

          [heads [pivot & tails]]
          (split-at pivot-index xs)
          
          xs-ex-pivot
          (concat heads tails)
           
          {lesser true, bigger false}
          (group-by #(< % pivot) xs-ex-pivot)]
      (concat (quick-sort lesser) 
              [pivot] 
              (quick-sort bigger)))))