private function key()
    {
        $offset = $this->tokens->key();
        $op = $this->tokens->getToken($offset - 1);

        if ($op
            && isset($this->tokens[$offset - 2])
            && ((DocLexer::T_COLON === $op[0])
                || (DocLexer::T_EQUALS === $op[0]))) {
            return $this->tokens->getValue($offset - 2);
        }

        return null;
    }