Trying to change a predefined value in a package

Question

I am trying to use the dnaseq package (documentation here: http://mirrors.linsrv.net/tex-archive/macros/latex/contrib/dnaseq/dnaseq.pdf)

Basically, it takes whatever you put between "\DNA!" and "!", organizes it in blocks of customizable length, and displays the text with a numbering on the left, starting from 1.

This package is useful for displaying easy-to-read DNA sequences, but for any reason, I would like it to start numbering from a negative number (actually my starting reference is in the middle of my sequence).

So I have to redefine the starting number to something negative. I'm not new to latex but I never tried to customize packages, this is why I request some help to :

-localize in code the variable to change (I think it's \@tempcntb=0)

-write a command to redefine this variable from my preamble (something like \def, not used to its syntax)

This package is relatively simple so I hope somebody will help me out.

Thank you anyway !

Answer 1

Please when asking question always provide a "MWE" complete document and say what the expected output should be, that makes it a lot easier to answer/test questions. this is an untested answer..

dnaseq defines

\def\DNA#1{%
   \def\@DNA@end{#1}\bgroup\ttfamily\DNAc@lcline
   \settoheight\dimen@{I}\advance\dimen@ by 1pt
   \edef\htst{\the\dimen@}%
   \def\struty{\rule[-.5pt]{\z@}{\htst}}%
   %% dnabase per line counter
   \count@=0
   %% block counter
   \@tempcnta=0
   %% total dnabase counter
   \@tempcntb=0
   \fboxrule=0pt \fboxsep=0pt
   \noindent\phantom{\DNAreserve}\llap 1\
   \@DNA
}

so you could for example put, after loading the package

\makeatletter
\def\DNA#1#2{%
   \def\@DNA@end{#2}\bgroup\ttfamily\DNAc@lcline
   \settoheight\dimen@{I}\advance\dimen@ by 1pt
   \edef\htst{\the\dimen@}%
   \def\struty{\rule[-.5pt]{\z@}{\htst}}%
   %% dnabase per line counter
   \count@=0
   %% block counter
   \@tempcnta=0
   %% total dnabase counter
   \@tempcntb=#1
   \fboxrule=0pt \fboxsep=0pt
   \advance\@tempcntb 1
   \noindent\phantom{\DNAreserve}\llap{\the\@tempcntb}\
   \advance\@tempcntb -1
   \@DNA
}
\makeatother

and use as ... \DNA{25}!....! with the counter being initialised from 25 rather than 0.