public function addRoll(Roll $roll)
 {
     if (!is_null($this->firstRoll) && !is_null($this->secondRoll)) {
         throw new TooManyRolls("Too Many Rolls");
     }
     if ($this->firstRoll && $this->firstRoll->numberOfPins() + $roll->numberOfPins() > 10) {
         throw new TooManyPins("Too Many Pins");
     }
     if (is_null($this->firstRoll)) {
         $this->firstRoll = $roll;
     } else {
         $this->secondRoll = $roll;
     }
 }