New command for multiple subsection

Question

I am writing an university report and I have to answer to a series of topics divided in points. I'd like to obtain a header like this 1.1.a,b,c. I have written this command to generate a single subsubsection referring to multiple points:

\documentclass{article}
\usepackage{lipsum}
\newcount\tmp

\newcommand{\multisub}[2][1]{%
\vspace{1.2\baselineskip plus .35\baselineskip minus.07\baselineskip}%
%\vspace{.5em}%
{\bf\thesubsection.%
\tmp=0%
\advance\tmp by 2%
%#1 max num
%#2 text
\loop%
    \advance\tmp by 1%
    \addtocounter{subsubsection}{1}%
    \alph{subsubsection},%
    \ifnum \tmp<#1%
\repeat%
\addtocounter{subsubsection}{1}%
\alph{subsubsection} #2\\%
}%
%    \vspace{.5em}%
\vspace{.8\baselineskip plus .07\baselineskip}%
{\normalfont\normalsize\bfseries}%
}%

\begin{document}
\section{First section}
\subsection{First subsection}
\lipsum[1]
\multisub[2]{First two points}
\lipsum[2]
\end{document}

The result is the following:

As you can see it has the form I'm looking for, but the spacing after the subsubsection header is wrong, as if the space after the header comes only after the first line of text.

How can I solve it? And how can I add this new subsection to the table of contents?

Thanks in advance.

Answer 1

Here is how you can define a \multisubsection command for coping with more subsections at a time.

\documentclass[oneside]{book} % oneside just to make a sensible picture
\usepackage{xparse}
\usepackage{kantlipsum}

\newcommand{\normalthesubsection}{%
  \renewcommand{\thesubsection}{\thesection.\alph{subsection}}%
}
\normalthesubsection

% add space for the composite numbers, adapt to your need
% increasing or decreasing 5em
\makeatletter
\renewcommand\l@subsection{\@dottedtocline {2}{3.8em}{5em}}
\makeatother

\ExplSyntaxOn
\NewDocumentCommand{\multisubsection}{mO{#3}m}
 {% #1 = number of subsections to span
  % #2 = optional short title
  % #3 = title
  \tl_clear:N \l_tmpa_tl
  \int_step_inline:nn { #1 - 1 }
   {
    \stepcounter{subsection}
    \tl_put_right:Nx \l_tmpa_tl { \alph{subsection}, }
   }
  \renewcommand{\thesubsection}{\thesection.\l_tmpa_tl\alph{subsection}}
  \subsection[#2]{#3}
  \normalthesubsection
 }
\ExplSyntaxOff

\begin{document}

\frontmatter

\tableofcontents

\mainmatter

\chapter{Title}

\section{Title}

\subsection{First point}

\kant[1]

\multisubsection{3}{Three more points}\label{a}

\ref{a}\kant[2]

\subsection{Another point}

\end{document}

Also \multisubsection{<n>}[Title for the toc]{Title in the document} is supported.

How does this work? When a \multisubsection is called, I step the relevant counter one time less than the argument specifies, collecting the “numbers” (in this case, letters) followed by a comma in \l_tmpa_tl; then I redefine \thesubsection to be as usual, but with the insertion of \l_tmpa_tl before the final number; the \subsection command called in the background will do the final stepping and print the full number. Finally, the standard meaning of \thesubsection is restored.