public function parseLabel()
    {
        $token = '';

        /**
         * Value to be returned.
         *
         * @var Token
         */
        $ret = null;

        /**
         * The value of `$this->last` where `$token` ends in `$this->str`.
         *
         * @var int
         */
        $iEnd = $this->last;
        for ($j = 1; $j < Context::LABEL_MAX_LENGTH && $this->last < $this->len; ++$j, ++$this->last) {
            if ($this->str[$this->last] === ':' && $j > 1) {
                // End of label
                $token .= $this->str[$this->last];
                $ret = new Token($token, Token::TYPE_LABEL);
                $iEnd = $this->last;
                break;
            } elseif (Context::isWhitespace($this->str[$this->last]) && $j > 1) {
                // Whitespace between label and :
                // The size of the keyword didn't increase.
                --$j;
            } elseif (Context::isSeparator($this->str[$this->last])) {
                // Any other separator
                break;
            }
            $token .= $this->str[$this->last];
        }

        $this->last = $iEnd;

        return $ret;
    }