expl3: lowercase sequence elements with line break breaks

Question

I have some short text (book author names) stored in a sequence. Some author names are long, and therefore I have to insert manually a line break (\\). In a specific case, I have to typeset these names completely lowercase.

Thanks to the help of Joseph Wright, the LowerCase works fine with text that contains the paragraph breaks (\\).

But now, I store the text in a sequence \g_sbmcpm_listofauthors_seq and then I create a new sequence with the lowercased text \g_sbmcpm_listofauthors_lowercase_seq using \seq_map_function:NN.

When I try to print the names with \seq_use:Nn I get compilation errors. If the names don't have \\ it works fine. But if there are some \; it only works when the string containing the \\ token is in the last position of the sequence. (It doesn't matter if I use \\, \par or \newline). I get an error message like this

! Undefined control sequence.
<inserted text> ... }{Author NameOne \\ TooLong}\\
                                                  \__xparse_start_expandable...
l.54     \printauthorslowercase

If instead of \\ in the \seq_use:Nn, I use a ; character the error looks like this:

! Undefined control sequence.
<argument> \\

l.54     \printauthorslowercase

Below is the complete code:

\documentclass{article}

\usepackage{xparse}
\usepackage{tikz}

\ExplSyntaxOn
\NewDocumentCommand \LowerCase { m }
  {
    \cs_set_eq:NN \__texnik_newline: \\
    \cs_set_protected:Npx \\ { \exp_not:o \\ }
    \tl_lower_case:n {#1}
    \cs_set_eq:NN \\ \__texnik_newline:
  }

\cs_new:Npn \add_lowerauthor #1
{
  \tl_set:No \l_tmpa_tl {\LowerCase{#1}}
  \seq_gput_right:No \g_sbmcpm_listofauthors_lowercase_seq {\l_tmpa_tl}
  \seq_log:N \g_sbmcpm_listofauthors_lowercase_seq
}


\seq_new:N \g_sbmcpm_listofauthors_seq
\DeclareDocumentCommand{\mainauthor} {m} {%
  \seq_put_right:Nn \g_sbmcpm_listofauthors_seq {#1}
}


\seq_new:N \g_sbmcpm_listofauthors_lowercase_seq
\NewDocumentCommand \printauthorslowercase { }
  {
    \seq_map_function:NN \g_sbmcpm_listofauthors_seq \add_lowerauthor
    \seq_use:Nn \g_sbmcpm_listofauthors_lowercase_seq  {;}
  }

\ExplSyntaxOff

%% This works
% \mainauthor{Author NameOne}
% \mainauthor{Author NameTwo}

%%% This also works
% \mainauthor{Author NameOne}
% \mainauthor{Author NameTwo \\ TooLong}

%%% This doesn't works
\mainauthor{Author NameOne \\ TooLong}
\mainauthor{Author NameTwo}

\begin{document}
\begin{tikzpicture}[overlay, remember picture]
  \node[align=left] (text1)
  {%
    \printauthorslowercase
  };
\end{tikzpicture}

\end{document}

Answer 1

I'm not sure I follow, but I don't see problems in passing \\ to \text_lowercase:n.

\documentclass{article}

\usepackage{xparse}
\usepackage{tikz}

\ExplSyntaxOn

\NewDocumentCommand{\mainauthor} {m}
 {
  \seq_put_right:Nn \g_sbmcpm_listofauthors_seq {#1}
 }

\NewDocumentCommand \printauthorslowercase { }
 {
  \seq_map_function:NN \g_sbmcpm_listofauthors_seq \sbmcpm_add_lowerauthor:n
  \seq_use:Nn \l_sbmcpm_listofauthors_lowercase_seq  {;}
 }

\seq_new:N \g_sbmcpm_listofauthors_seq
\seq_new:N \l_sbmcpm_listofauthors_lowercase_seq

\cs_new_protected:Npn \sbmcpm_add_lowerauthor:n #1
 {
  \seq_put_right:Nn \l_sbmcpm_listofauthors_lowercase_seq { \text_lowercase:n {#1} }
 }

\ExplSyntaxOff

\mainauthor{Author NameOne \\ TooLong}
\mainauthor{Author NameTwo}

\begin{document}

\begin{tikzpicture}[overlay, remember picture]
  \node[align=left] (text1)
  {%
    \printauthorslowercase
  };
\end{tikzpicture}

\end{document}