Needs arbitrarily long tests... things like...

(let [expected (range (rand-int 200) (+ 200 (rand-int 200))
      n (count expected)
      [a b] (map sort (split-at (rand-int n) (shuffle expected)))]
      
      (testing (format "a: %s b: %s" a b)
        (is (= expected (mergesorted a b)))))

I just wanted to add a simple fork that shows off what the Project Euler question was hinting towards. The iterative solution becomes slow beyond a million or so.

Ok, this should fix jetjake's code so it correctly tests. Also cleaned up formatting and spelling.