code: public function setClosingBalanceClass($closingBalanceClass)
      {
          if (!is_callable($closingBalanceClass) && !class_exists($closingBalanceClass)) {
              throw new \InvalidArgumentException('$closingBalanceClass must be a valid classname or a PHP callable');
          }
  
          $this->closingBalanceClass = $closingBalanceClass;
          return $this;
      }
docs: Set the classname of the closing balance class or callable that returns an object that
  implements the BalanceInterface.
  
  The callable is not passed any arguments.
  
  $reader->setClosingBalanceClass(function () {
  return new My\Balance();
  });
  
  @param string|callable $closingBalanceClass
  @return $this