private function equivalenceHash( $elementName, $linkName, $linkRole, $fromHref, $toHref, $order = 1, $weight = null, $preferredLabel = null )
	{
		if ( ! is_numeric( $order ) ) $order = null;
		if ( ! is_numeric( $weight ) ) $weight = null;

		return hash( "sha256", "$elementName-$linkName-$linkRole-$fromHref-$toHref-$order-$weight-$preferredLabel" );
	}