I would suggest defining a variant of the traditional \section commands to make the code more readable (rather than adding the markup as part of the section title). It will also allow you to modify things later on if you wish to change the syntax.
\documentclass{article}
\usepackage{xparse,amssymb}
\makeatletter
\NewDocumentCommand{\hardsection}{s o m}{{%
\renewcommand{\@seccntformat}[1]{\hardsec\csname the##1\endcsname\quad}%
\IfBooleanTF{#1}
{% \hardsection*
\IfNoValueTF{#2}
{\section*{#3}}% \hardsection*{..}
{\section*[#2]{#3}}% \hardsection*[..]{..}
}
{% \hardsection
\IfNoValueTF{#2}
{\section{#3}}% \hardsection{..}
{\section[#2]{#3}}% \hardsection[..]{..}
}
}}
\NewDocumentCommand{\hardsubsection}{s o m}{{%
\renewcommand{\@seccntformat}[1]{\hardsec\csname the##1\endcsname\quad}%
\IfBooleanTF{#1}
{% \hardsubsection*
\IfNoValueTF{#2}
{\subsection*{#3}}% \hardsubsection*{..}
{\subsection*[#2]{#3}}% \hardsubsection*[..]{..}
}
{% \hardsubsection
\IfNoValueTF{#2}
{\subsection{#3}}% \hardsubsection{..}
{\subsection[#2]{#3}}% \hardsubsection[..]{..}
}
}}
\newcommand*{\hardsec}{\llap{\hardsecsymbol\hspace{\hardsecspace}}}
\newcommand{\hardsecsymbol}{$\blacklozenge$}
\newlength{\hardsecspace}\setlength{\hardsecspace}{20pt}
\makeatother
\setcounter{secnumdepth}{2}
\begin{document}
\section{The first section}
\subsection{Beginnings}
This bit is for all-comers.
\hardsubsection{Harder stuff}
Here's some harder stuff
\hardsection{The second section}
This entire section is so hard it's in Latin. Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
\section{The third section}
\subsection{Middlings}
Bits of this subsection are accessible.
\subsubsection{For everyone}
This part is, for example.
\subsubsection[abc]{\hardsec For the (self-)chosen few}
But this part is tricky
\end{document}
The above definitions of \hardsection (and friends) adjust \@seccntformat - the printing mechanism for the sectional heading numbers. It is assumed that the publisher may change peripheral parts of sectional definitions, but might leave \@seccntformat untouched - essential for making the above suggestion work.
The insertion of \hardsec is font-specific, so you'll see it adjust based on whether you have a \hardsection or a \hardsubsection. This may not be ideal but could be changed. Additionally, instead of using a font-specific spacing \hardsecspace, use a fixed-with space in points, say. Other alignment options also exist, of course.
The xparse use of the definition for \hardsection is to allow you total control over how things are processed. The contents of the new command is grouped, making the redefinition of \@seccntformat temporary.
amssymb provides \blacklozenge which is moderator-like.
\newcommand{\hardsection}{\mbox{\hss$\diamondsuit$\hspace{xxx}}}wherexxxis the width of the space occupied by the section number. the\hsseffectively zeros out the width of this construct, and shoves the diamond off to the left of the section number. (you will probably want to increase the\hspaceamount a bit so the diamond isn't tight up against the section number.) – barbara beeton Feb 02 '15 at 18:58The marks are "dangerous curves" signs, they're in the outer margin, and there can be from zero to three of them.
He explains that when you first read the book, you should read only those paragraphs with no such signs - this gives you the structure. On your second read you should probably read most of the "one sign" paragraphs. Then you'll know which of the harder paragraphs will be of interest to you. The code is in the book.
– Jamie Hanrahan Feb 03 '15 at 07:13