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