public function parseUnknown()
    {
        $token = $this->str[$this->last];
        if (Context::isSeparator($token)) {
            return null;
        }

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

        return new Token($token);
    }