如果两个节标题彼此相连，即它们之间没有其他文本或其他内容，我想在它们之间添加额外的垂直空间。是否有一些条件 if 命令，例如使用 if@aftersection...？
\documentclass[12pt, a4paper]{memoir}
\usepackage[utf8]{inputenc}
\begin{document}
\chapter{Chapter title}
\section{A first section title}
\section{A second section title}
Just some text. There should be a conditional vspace between the first
section and the second section if there is nothing else between them.
\section{A third section}
Some more text here.
\section{A fourth section}
Some more text.
\end{document}
答案1
一般来说，你必须适应所有可能的参数组合memoir的\section。具体来说，它需要两个可选参数（用于目录和运行标题）。除此之外，还可以扫描以查看下一个标记是否正在\section使用\@ifnextchar：
\documentclass{memoir}
\usepackage{xparse}
\let\oldsection\section
\makeatletter
\RenewDocumentCommand{\section}{s o o m}{%
\IfBooleanTF{#1}
{\oldsection*{#4}}
{\IfValueTF{#2}
{\IfValueTF{#3}
{\oldsection[#2][#3]{#4}}
{\oldsection[#2]{#4}}}
{\oldsection{#4}}}%
\@ifnextchar\section{\vspace{5\baselineskip}}{}%
}
\makeatother
\begin{document}
\tableofcontents
\chapter{Chapter title}
\section{A first section title}
\section{A second section title}
Just some text. There should be a conditional \verb|\vspace| between the first
section and the second section if there is nothing else between them.
\section{A third section}
Some more text here.
\section{A fourth section}
Some more text.
\end{document}
这是另一个选择：
定义一个获取三个参数的宏\ifnextcommandis{<this>}{<true>}{<next>}。指定<this>和<true>，并假定宏后面的内容将是<next>宏：
\newcommand{\ifnextcommandis}[3]{%
\def\thiscommand{#1}% Store this command
\def\nextcommand{#3}% Store next command
\ifx\nextcommand\thiscommand\relax
#2%
\fi
\nextcommand}% Re-insert next command
\let\oldsection\section
\RenewDocumentCommand{\section}{s o o m}{%
\IfBooleanTF{#1}
{\oldsection*{#4}}
{\IfValueTF{#2}
{\IfValueTF{#3}
{\oldsection[#2][#3]{#4}}
{\oldsection[#2]{#4}}}
{\oldsection{#4}}}%
\ifnextcommandis{\section}{\vspace{5\baselineskip}}%
}