Derivative package: does it include square brackets with comment?

The source file of user manual derivative.pdf is derivative.tex. It uses a variant underbracket:

documentclass{article}
usepackage{mathtools}
usepackage{unicode-math} % this loads "Latin Modern Math" font, which changes the line width of underbracket
usepackage{xparse}

ExplSyntaxOn
DeclareDocumentCommand{low}{ O{0,0} m m }
{
    seq_set_from_clist:Nn l__low_seq {#1}
    
    mskip -seq_item:Nn l__low_seq {1} mu
    underbracket
    {
        mskip seq_item:Nn l__low_seq {1} mu
        vphantom{f}
        #2
        mskip seq_item:Nn l__low_seq {2} mu
    }
    sb{text{#3}}
    mskip -seq_item:Nn l__low_seq {2} mu
}
ExplSyntaxOff

begin{document}
    begin{equation*}
    low{
        low{
            low{4gg}{big} +
            low[2,2]{2ff}{low}
        }{long} +
        low{ 
            low[1,2]{4c}{big} +
            low[1,1]{3a}{low}
        }{short}
    }{positve} -
    low{
        low{
            low[3,3]{5ee}{big} -
            low[2,2]{3hh}{low}
        }{long} -
        low{
            low[1,1]{5d}{big} -
            low[2,2]{2b}{low}
        }{short}
    }{negative}
    end{equation*}
end{document}