private function setProperty($element, $key, $value)
{
if (!in_array($element, $this->multiplePropertiesForElementAllowed) && isset($this->definedElements[$element])) {
throw new Exception('You can only set "' . $element . '" once.');
}
// we define that we set this element
$this->definedElements[$element] = true;
// adding property
$this->properties[] = array('key' => $key, 'value' => $value);
}