public function getKey($offset = null)
    {
        if (null === $offset) {
            $offset = $this->offset;
        }

        if ((null !== ($op = $this->getId(--$offset)))
            && ((DocLexer::T_COLON === $op)
                || (DocLexer::T_EQUALS === $op))) {
            return $this->getValue(--$offset);
        }

        return null;
    }