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