public function parseWhitespace()
    {
        $token = $this->str[$this->last];

        if (! Context::isWhitespace($token)) {
            return null;
        }

        while (++$this->last < $this->len && Context::isWhitespace($this->str[$this->last])) {
            $token .= $this->str[$this->last];
        }

        --$this->last;

        return new Token($token, Token::TYPE_WHITESPACE);
    }