public function addUpgraded(DOMElement $el, $property) {
		if ( !is_array($property) ) {
			$property = array($property);
		}

		// add element to list of upgraded elements
		if ( !$this->upgraded->contains($el) ) {
			$this->upgraded->attach($el, $property);
		} else {
			$this->upgraded[$el] = array_merge($this->upgraded[$el], $property);
		}
	}