Write a function that takes a sequence of integers and returns a sequence of "intervals". Each interval is a a vector of two integers, start and end, such that all integers between start and end (inclusive) are contained in the input sequence.

(ns live.test
  (:require [cljs.test :refer-macros [deftest is run-tests]]))
  
(defn intervals [c]
  (let [s (set c)]
    (map (juxt first last)
      (take-nth 2 (partition-by #(contains? s %)
                     (and (seq s)
                       (range (apply min s) (inc (apply max s)))))))))

(deftest intervals-test
  (is (= (intervals [1 2 3]) [[1 3]]))
  (is (= (intervals [10 9 8 1 2 3]) [[1 3] [8 10]]))
  (is (= (intervals [1 1 1 1 1 1 1]) [[1 1]]))
  (is (= (intervals []) []))
  (is (= (intervals [19 4 17 1 3 10 2 13 13 2 16 4 2 15 13 9 6 14 2 11])
         [[1 4] [6 6] [9 11] [13 17] [19 19]])))
         
(run-tests)