code: private function handleFirstDigit(Context $context)
      {
          if (self::checkDigit($context)) {
              $this->result .= $context->current();
              $context->next();
          } else {
              throw $context->createException("Invalid number format");
          }
      }
docs: 1 文字目の数字を読み込みます.
  
  @param Context $context