/**
* Initializes the set using the given elements.
*
* In case element type is set, it is not possible to add an object of
* another type to the set.
*
* @param null|string $elementType Type of the elements of the set
* @param array $elements Initial elements of the set
*/
public function __construct(string $elementType = null, array $elements = array())
{
// set element type
$this->elementType = $elementType;
// init set with given elements
parent::__construct($elements);
}
