我想定义/创建一个与紧随其后的第一段相关的悬挂小节（在界限内）；也就是说，整个段落缩进。我试图在这个 MWE 中提供我的梦想的图片/展示。我的梦想有可能实现吗？
\documentclass[10pt]{book}
%%==============================================================================
\usepackage{etoolbox}
\usepackage{enumitem}
\usepackage{amsmath,amsthm}
\usepackage{kantlipsum}
%%-----------------------------------------------------------------------------
\makeatletter
\newcommand\qsection{\suppressfloats[t]\@startsection% floats not to move backwards
{section}% % name
{1}% % level--section heading
{0pt}% % no indentation from left margin
{-1.0\baselineskip minus\parskip}% % beforeskip--suppress par indentation
{0.5\baselineskip}% % afterskip--display heading
{\normalfont\large\bfseries\raggedright}}% style
%%------------------------------------------------------------------------------
\newcommand\qsubsection{\@startsection
{subsection}% % name
{2}% % level--subsection heading
{0pt}% % no indentation from left margin
{-0.5\baselineskip plus0.2\baselineskip minus0.2\baselineskip}%
{-0.10ex}% % afterskip (run-in)
{\normalfont\bfseries\raggedright}}% % style
\makeatother
%%------------------------------------------------------------------------------
\renewcommand\thesection{\Alph{section}.}
\renewcommand\thesubsection{\arabic{subsection}.}
%%------------------------------------------------------------------------------
%% special theoremstyle environment for trial subsection definition
%%------------------------------------------------------------------------------
\newtheoremstyle{fakesub}
{0.5\baselineskip plus0.2\baselineskip minus0.2\baselineskip}%
{0pt}%
{\normalfont}%
{0pt}%
{\bfseries}%
{}%
{0em}%
{\llap{\makebox[\defsep][l]{%
\thmname{#1}~\thmnumber{#2}\if\relax\detokenize{#3}\relax.\fi}}%
\thmnote{~{\normalfont(#3)}}}
%%------------------------------------------------------------------------------
\theoremstyle{fakesub}
\newtheorem{defn}{Definition}[chapter]
\newtheorem{fakesubsection}[defn]{\!\!}% ``fake'' subsection definition
%%------------------------------------------------------------------------------
\newlength\defsep
\setlength{\defsep}{4.5ex}%
%%
\makeatletter
\AtBeginEnvironment{fakesubsection}{%
\refstepcounter{subsection}
\renewcommand\thedefn{\arabic{subsection}}
\patchcmd{\@thm}{\trivlist}{\list{}{\leftmargin=\defsep}}{}{}
\patchcmd{\@endtheorem}{\endtrivlist}{\endlist}{}{}
}
\makeatother
%%------------------------------------------------------------------------------
\newcommand*{\fmap}[1]{\ensuremath{#1\mkern1mu\textnormal{:}}\;}
\begin{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\chapter{Abelian Groups}
%%------------------------------------------------------------------------------
I would like to define a subsection to look like a hang section (within bounds) with respect to the first paragraph following the subsection; that is, the whole paragraph be indented. I prefer the ``fake'' subsection is $\mathbf{1}$\textbf{.} under section~\textbf{\ref{02VectorSpaces}} to that of the ``real'' one under section~\textbf{\ref{01VectorSpaces}} ...
%%------------------------------------------------------------------------------
\qsection{Vector Spaces}\label{01VectorSpaces}
%%--------
My aim is to make this ``real'' subsection $\mathbf{1}$\textbf{.} to display like the ``fake'' one under section~\textbf{\ref{02VectorSpaces}}.
\qsubsection{}\label{01RealSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication.
%%------------------------------------------------------------------------------
\qsection{Vector Spaces}\label{02VectorSpaces}
%%--------
This is ``fake'' subsection $\mathbf{1}$\textbf{.} under section~\textbf{\ref{02VectorSpaces}} ...
\begin{fakesubsection}\label{01FakeSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication. %(Second and subsequent paragraphs after the ``fake'' subsection are displayed in normal fashion.)
\end{fakesubsection}
%%------------------------------------------------------------------------------
\end{document}
%%==============================================================================
答案1
我使用定制的枚举方法（Bernard 的建议）来回答这个问题。我希望以下 MWE 能够足够清楚地表达我提出这个问题的初衷。
%%-------------------------------------------------------------------
%% a hang subsection within margins using a custom enumerate
%%--------------------------------------------------------------------
\documentclass[10pt]{book}
%%--------------------------
\usepackage{amsmath,amsthm}
\usepackage{enumitem}
\usepackage{xparse}
\makeatletter
\newcommand\qsection{\suppressfloats[t]\@startsection%
{section}%
{1}%
{0pt}%
{-1.0\baselineskip minus\parskip}%
{0.5\baselineskip minus\parskip}%
{\normalfont\large\bfseries\raggedright}}%
\makeatother
\renewcommand\thesection{\Alph{section}.}
%%-------------------------------------------------------------------
\newlength{\hangwidth} \newlength\afterskip
%%-------------------------------------------------------------------
\ExplSyntaxOn
\NewDocumentEnvironment{hsubsection}{O{} O{.5} O{}}%
{%%
\setlength{\afterskip}{-#2\baselineskip}
\renewcommand\thesubsection{\arabic{subsection}}
%%------------
\tl_if_empty:nTF{#1}%
{% %% no title header
\refstepcounter{subsection}
\settowidth{\hangwidth}{\bfseries\thesubsection.}
\def\hanglabel{\bfseries\thesubsection.}
}
{%
\tl_if_empty:nTF{#3}%
{% %% unnumbered title header
\settowidth{\hangwidth}{\bfseries#1}
\def\hanglabel{\bfseries#1}
}
{% %% numbered title header
\refstepcounter{subsection}
\settowidth{\hangwidth}{\bfseries\thesubsection\hspace{.5em}#1}
\def\hanglabel{\bfseries\thesubsection\hspace{.5em}#1}
}
}
%%-------------------------------------------------------------------
\begin{hangSubsection}
\item
}
{%
\end{hangSubsection}
\vspace{\afterskip}
\par%
}
\ExplSyntaxOff
%%-------------------------------------------------------------------
\newlist{hangSubsection}{enumerate}{1}%
%%--------------------------------
\setlist[hangSubsection]{%
label=\hanglabel,
ref=\thesubsection,
align=left,
leftmargin={\dimexpr \hangwidth + 1.0em},
labelwidth=*,
topsep=1\baselineskip plus0.2\baselineskip minus0.2\baselineskip,
itemsep=0pt,
before=\normalfont\normalsize
}
%%-------------------------------------------------------------------
\newcommand*{\fmap}[1]{\ensuremath{#1\mkern1mu\textnormal{:}}\;}
%%===================================================================
\begin{document}
%%-------------------------------------------------------------------
\chapter{Abelian Groups}
%%----------------------
An attempt to define a hang subsection with the first paragraph following it all indented, and within the boundaries of the left and right margins.
%%
\qsection{Vector Spaces}\label{01VectorSpaces}
We will attempt to show hang subsections with no title header, with unnumbered short title header, and with numbered short title header.\par%
It may be prudent to use the normal ``display'' or ``run-in'' sectioning definitions for for subsections with long title headers.
\begin{hsubsection}[][.7]\label{01FakeSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication.
\end{hsubsection}
下一节带有未编号的短标题标题：
%%
\begin{hsubsection}[Claim][.7]\label{02FakeSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication.
\end{hsubsection}
这是另一个没有标题标题的编号小节……
\begin{hsubsection}\label{03FakeSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication.
\end{hsubsection}
这是一个带有简短标题标题的编号小节...
%%
\begin{hsubsection}[Claim][.7][Yes]\label{04FakeSubsection}
Two vector spaces are \emph{isomorphic} if there is a bijection $\fmap{\varphi}L\to L'$ such that $\varphi(\lambda{a} +\mu{b}) = \lambda\varphi(a)+\mu\varphi(b)$ for all $\lambda$, $\mu$, $a$ and $b$; that is, $\varphi$ preserves the operations of addition and multiplication.
\end{hsubsection}
\end{document}