From e0c6872cf40896c7be36b11dcc744620f10adf1d Mon Sep 17 00:00:00 2001
From: Norbert Preining <norbert@preining.info>
Date: Mon, 2 Sep 2019 13:46:59 +0900
Subject: Initial commit

---
 macros/latex/contrib/sphack/sphack-doc.tex | 115 +++++++++++++++++++++++++++++
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 create mode 100644 macros/latex/contrib/sphack/sphack-doc.tex

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diff --git a/macros/latex/contrib/sphack/sphack-doc.tex b/macros/latex/contrib/sphack/sphack-doc.tex
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+\documentclass[pagesize=auto]{scrartcl}
+
+\usepackage{fixltx2e}
+\usepackage{etex}
+\usepackage{lmodern}
+\usepackage[T1]{fontenc}
+\usepackage{textcomp}
+\usepackage{microtype}
+\usepackage{hyperref}
+
+\newcommand*{\mail}[1]{\href{mailto:#1}{\texttt{#1}}}
+\newcommand*{\pkg}[1]{\textsf{#1}}
+\newcommand*{\cs}[1]{\texttt{\textbackslash#1}}
+\makeatletter
+\newcommand*{\cmd}[1]{\cs{\expandafter\@gobble\string#1}}
+\makeatother
+
+\addtokomafont{title}{\rmfamily}
+
+\title{The \pkg{sphack} package\thanks{This manual corresponds to \pkg{sphack.sty}~v1.0, dated~22 May 1998.}}
+\author{Oliver Pretzel\thanks{\mail{o.pretzel@ic.ac.uk}}}
+\date{22 May 1998}
+
+
+\begin{document}
+
+\maketitle
+
+\noindent
+Standard \LaTeX\ uses the macros \cmd{\@bsphack}, \cmd{\@esphack}, and \cmd{\@Esphack}, for
+inserts into the text that should be invisible. So, for instance a space
+before and after a \cmd{\label} command should not result in two spaces in the
+output.
+
+\LaTeX\ deals with this as follows
+%
+\begin{itemize}
+\item in maths mode do nothing,
+\item in horizontal mode restore the space factor,
+\item if the last thing on the list was a space
+  add \cmd{\ignorespaces} (and (\cmd{\global})\cmd{\ignore\-true} at the end of an environment),
+\item in vertical mode do nothing.
+\end{itemize}
+
+Doing nothing in maths mode is (nearly) harmless because maths mode does its
+own spacing (and anyway hidden commands will usually appear only at the start
+or end of maths).
+
+Doing nothing in vertical mode is not harmless. Many invisible commands such
+as \cmd{\index} may insert delayed write commands into \TeX's output (so that page
+numbers are correctly calculated). These commands can cause vertical space to
+accumulate, and may cause a page break; \cmd{\index} is a particular problem in
+\LaTeX2.09 since it inserts a delayed write if an index is actually being built
+(\cmd{\makeindex} in preamble) but does nothing otherwise. That can change the page
+breaks in a document.
+
+It is not possible to solve this problem completely in \LaTeX\ because \TeX\ does
+not remove things from the main vertical list once they have been
+contributed. So \LaTeXe\ makes \cmd{\index} insert something into the vertical
+list whether the index is being written or not. That has the virtue of
+consistancy, but is far from ideal. For instance, an \cmd{\index} immediately after
+an \cmd{\item} can cause the page to break between the item label and content.
+
+\pagebreak[2]
+
+The code in the \pkg{sphack} package remedies this fault and other common anomalies so that commands
+are nearly always invisible in vertical mode as well. It works as follows:
+%
+\begin{itemize}
+\item Rename \LaTeX's dimension \cmd{\@savsk} to \cmd{\@savdim} set a new skip \cmd{\@savsk}
+  (because we need a true skip and a dimension in vertical mode)
+
+\item \cmd{\@bsphack} (at start of invisible command)
+  \begin{itemize}
+  \item in hmode (non-math)
+    \begin{enumerate}
+    \item store spacefactor in \cmd{\@savsf}
+    \item store lastskip in \cmd{\@savsk} (used to test whether space already present)
+    \end{enumerate}
+
+  \item in vmode
+    \begin{enumerate}
+    \item store lastpenalty in \cmd{\@savsf}
+    \item store lastskip in \cmd{\@savsk} (used for movement)
+    \item store previous depth  in \cmd{\@savdim}
+    \item skip back \cmd{\lastskip}
+    \end{enumerate}
+  \end{itemize}
+
+\item \cmd{\@esphack} (at end of invisible command)
+  \begin{itemize}
+  \item in hmode (non-math)
+    \begin{enumerate}
+    \item set spacefactor = \cmd{\@savsf}
+    \item if \cmd{\@savsk} > \cmd{\z@} \cmd{\ignorespaces}
+    \end{enumerate}
+
+  \item in vmode
+    \begin{enumerate}
+    \item if in a label, or just after section heading, or if \cmd{\@nobreak}\\
+      insert infinite penalty, (to prevent a page break)\\
+      else insert penalty \cmd{\@savsf}\\
+      endif
+    \item set previous depth = \cmd{\@savdim}
+    \item skip \cmd{\@savsk}
+    \end{enumerate}
+  \end{itemize}
+\end{itemize}
+%
+Just as in standard \LaTeX, \cmd{\@Esphack} is \cmd{\@esphack} + (\cmd{\global})\cmd{\@ignoretrue}.
+
+It is not necessary to change any of the label or index macros to fit with
+this code.
+
+\end{document}
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