(define play-with-turns
  (lambda (game-state player)
    (display-game-state game-state)
    (cond 
      ((over? game-state) (announce-winner player))
      ((equal? player 'human) 
       (play-with-turns (human-move game-state) 'computer))
      ((equal? player 'computer) 
       (play-with-turns (computer-move game-state) 'human))
      (else (error "player wasn't human or computer:" player)))))