code: public function setAccountClass($accountClass)
      {
          if (!is_callable($accountClass) && !class_exists($accountClass)) {
              throw new \InvalidArgumentException('$accountClass must be a valid classname or a PHP callable');
          }
  
          $this->accountClass = $accountClass;
          return $this;
      }
docs: Set the classname of the account class or callable that returns an object that
  implements the AccountInterface.
  
  The callable is passed the account number as a parameter. Example:
  
  $reader->setAccountClass(function ($accountNumber) {
  return new My\Account();
  });
  
  If the callable returns null, statements for the account will be skipped.
  
  @param string|callable $accountClass
  @return $this