/**
* @param Snak a snak with the same property
* @throw Exception
*/
public function setMainSnak(Snak $snak)
{
if (!$this->mainSnak->getPropertyID()->equals($snak->getPropertyID())) {
throw new Exception('Different property id!');
}
$this->mainSnak = $snak;
$this->changes['mainsnak'] = $snak->toArray();
}