Collection::merge

public function merge(?self $collection): self
{
	if (is_null($collection))
	{
		return $this;
	}

	if (get_class($this) !== get_class($collection))
	{
		throw new ArgumentException(
			'Invalid object class ' . get_class($collection) . ' for merge, ' . get_class($this) . ' expected .'
		);
	}

	foreach ($collection as $item)
	{
		$this->add($item);
	}

	return $this;
}