macro: call parameter of level above - double pound ##1 doesn't work

Question

Once again, I have a macro problem. It works almost. I spend a lot of time googling about \expandafter, \noexpand, \unexpanded and others. I used some of them, but nothing lead to the desired result.

If you find a better title for this problem, change it. I used the best, I could think of.

The following is my code:

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[german]{babel}
\usepackage{graphicx}
\usepackage{environ}



\makeatletter
%%%Liederbuch. This works!!!
\NewEnviron{Liederbuch}[1]{
\xdef\LB@my@temp{
\noexpand\newcommand{\csname LH#1\endcsname}[2]{
\unexpanded\expandafter{\BODY}
}
}
\aftergroup\LB@my@temp
}

%%%%%%%%%%%%%%%%%
%The problem starts in the following Environ:

%If I replace `##1` and `##2` with constants, it works, but therefore it fails the basic intention of this effort.

%To see, what should happen, have a look at `\begin{document}`
%%%%%%%%%%%%%%%%%

%%%Lied. This doesn't work!!!
\NewEnviron{Lied}[2]{%
    \ifnum\numexpr#2=\numexpr##2 % ##2 doesn't work; i.e. "1" instead of ##2 works
        \ifnum\pdfstrcmp{#1}{##1}=0 % ##1 doesn't work; same as above
            \BODY
        \fi
    \fi
}
\makeatother %residual of some tries with xdef and \aftergroup

%%%%%%%%%%%%%%%%%    
%Apparently, the following code is super clean. That is the basic idea.
%%%%%%%%%%%%%%%%%    

% This is a Liederbuch
\begin{Liederbuch}{songbook}
\begin{Lied}{t}{1}
Song 1 par t
\end{Lied}
\begin{Lied}{n}{2}
Song 2 par n
\end{Lied}
\end{Liederbuch}


% LHsong calls "the environment" This works!!!
\newcommand{\LHsong}[3][n]{
\csname LH#2\endcsname[#1]{#3}
}


\begin{document}

%This should produce Song 1 par t
\LHsong[t]{songbook}{1}

%This shouldn't produce anything
\LHsong[n]{songbook}{1}

%This should produce Song 2 par n
\LHsong{songbook}{2}

%This shouldn't produce anything
\LHsong[t]{songbook}{2}

%This should also produce Song 1 par t
\LHsongbook{t}{1}

\end{document}

The Liederbuch expands to the following code which isn't part of my code

% it expands to:    %not part of the code
%Liederbuch
\newcommand{\csname LHsongbook\endcsname}[2]{
%Lied 1
    \ifnum1=\numexpr##2 %\numexpr#2 expanded to 1; ##2 should be par2 of newcommand
        \ifnum\pdfstrcmp{t}{##1}=0 %#1 expanded to t
            Song 1 par t
        \fi
    \fi
%Lied 2 - same here different values
    \ifnum2=\numexpr##2
        \ifnum\pdfstrcmp{n}{##1}=0
            Song 2 par n
        \fi
    \fi
}

I stole the Liederbuch-Environ from one of my other questions. The proof, that this works, can be seen by the change of ##x to constants. I suppose, that Lied should look similar, but I couldn't figure out, how.

The intention is, to have environment Liederbuch create a [perhaps series of] custom Lied environment[s] in which an invocation of \LHsong will produce output from Lied if the arguments match certain specifications.

Answer 1

I don't know if this is precisely what you want, but this does seem to work. I loaded etoolbox for convenience (\csgdef, \csxdef, \csuse).

\documentclass{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{etoolbox}
\usepackage{environ}

\newenvironment{Liederbuch}[1]
  {\def\liederbuchtmp{#1}\csgdef{LH#1}##1##2{\csuse{lied;#1;##1;##2}}}
  {}
\NewEnviron{Lied}[2]{\csxdef{lied;\liederbuchtmp;#1;#2}{\expandonce\BODY}} % perhaps trimming spaces would be important
\newcommand*\LHsong[3][n]{\csuse{lied;#2;#1;#3}}

\begin{Liederbuch}{songbook}
\begin{Lied}{t}{1}
  Song 1 par t
\end{Lied}
\begin{Lied}{n}{2}
  Song 2 par n
\end{Lied}
\end{Liederbuch}

\begin{document}

% This should produce Song 1 par t
\LHsong[t]{songbook}{1}

% This shouldnt produce anything
\LHsong[n]{songbook}{1}

% This should produce Song 2 par n
\LHsong{songbook}{2}

% This shouldnt produce anything
\LHsong[t]{songbook}{2}

% This should also produce Song 1 par t
\LHsongbook{t}{1}

\end{document}