public function getVersion() {
        if ($this->version === null) {
            foreach ($this->mapping['properties'] as $name => $property) {
                if (isset($property['version']) && $property['version']===true) {
                    return $this->version = new PropertyMapping($this, $property);
                }
            }
            $this->version = false;
        }
        return $this->version ?: null;
    }