Indent text without using trivlist

Question

Quite a lot environments which change margins (e.g. center, quote, addmargin from KOMA) use internally trivlist. This has some drawbacks, e.g., when the list is directly between two sectioning commands:

\documentclass{article}
\begin{document}
\section{a section}
%a
\begin{quote}
\subsection*{An important quote}
blalbla
\end{quote}
\end{document}

e.g., gives the well known and feared error:

! LaTeX Error: Something's wrong--perhaps a missing \item.

So I tried to extract from trivlist the code that does the indentation and drop all the label related code and came up with the following code. My question is:

Is there some obvious flaw?

(It looks so simple that I wonder why it doesn't exist yet!)

\documentclass{article}
\usepackage{lipsum}

\makeatletter
\newenvironment{indentation}[2][0pt]%#1=right indentation
  {\par
   \begingroup
   \global\advance\@listdepth\@ne  
   \leftmargin=#2\relax
   \rightmargin=#1\relax
   \advance\linewidth -\rightmargin\relax
   \advance\linewidth -\leftmargin
   \advance \@totalleftmargin \leftmargin
   \parshape\@ne \@totalleftmargin \linewidth
   \@setpar{{\@@par}}}
  {\global\advance\@listdepth\m@ne\endgroup\par}
\makeatother

\begin{document}
\section{abc}
  \lipsum[1]
   \[a=b\]
\begin{itemize}
\item blabla
\end{itemize}


\section{abc}
\begin{indentation}[\leftmargini]{\leftmargini}
\subsection{abc}
 \lipsum[1]
 \begin{itemize}
  \item blub
   \begin{enumerate}
    \item Does it work?
   \end{enumerate} 
 \end{itemize}
 \[a=b\]
 \lipsum[2-3]

 \begin{indentation}{0pt}
  \centering
  \lipsum[1]
 \end{indentation}
\end{indentation}

\end{document}

Edit

I take back the "doesn't exist yet". I just found on CTAN http://www.ctan.org/tex-archive/macros/latex209/contrib/misc/indent.sty, which contains an almost identical definition (even with the same name). The only difference is that my code changes \@listdepth as I wanted to keep the relation of indent for nested lists.

Answer 1

If you remove the % from line number 4 of your code, the error goes away. A \section always has to contain some text.