nested_from_swap.clj

(def a [1 2])

(apply conj a [3 4])
;; [1 2 3 4]

(def a (atom [1 2]))

(swap! a conj [3 4])
;; [1 2 [3 4]]
;; According to the docs, swap! Atomically swaps the value of atom to be:
;; (apply f current-value-of-atom args). Note that f may be called
;; multiple times, and thus should be free of side effects.  Returns
;; the value that was swapped in.

;;  So, why does a have a nested vector in it?