public function setExtend($extendingSection, $extendedSection = null)
    {
        if ($extendedSection === null && isset($this->_extends[$extendingSection])) {
            unset($this->_extends[$extendingSection]);
        } else if ($extendedSection !== null) {
            $this->_extends[$extendingSection] = $extendedSection;
        }
    }