(function evolve generation parent

  (print (pad-left " " 3 generation) " " (... str parent) " " (fitness parent))

  (return-when (perfect-fit? parent))

  (let children (times c #(mutate parent))
       fittest  (max-by fitness (... vec parent children)))

  (recur (inc generation) fittest))

(evolve 1 (times tsize rand-char))