如何创建和使用像 TeXbook 中那样的“仅适用于巫师”符号？

Question

如何创建一个像 TeXbook 中用于指定仅供巫师使用的材料的符号，如下图红色圆圈所示，只用任意英文字母而不是锯齿形（我希望能够临时确定要显示哪个字母）？以及如何以与 TeX 书中类似的方式将此符号插入我选择的段落开头？

Answer 1

如今，你会在 Ti 中做出类似的事情钾Z. 我并不经常使用它，所以肯定会有更好的方法，但这里有一个想法：

\documentclass{article}

\usepackage{tikz}

\newcommand{\mydangersymbol}[1]{%
      \begin{tikzpicture}[baseline=(x.base)]
         \draw[rounded corners=.01em] (-.05em,-1.3em)rectangle(.05em,.9em);
         \draw[fill=white,rounded corners=1] (0,.8em)--(.8em,0)--(0,-.8em)--(-.8em,0)--cycle;
         \draw[very thick,line cap=round](-.3em,-1.3em)--(.3em,-1.3em);
         \node(x) at (0,0em) {\normalfont\sffamily\small#1};
      \end{tikzpicture}%
}

\newenvironment{mydanger}[1]{%
  \par\medskip\noindent
  \sbox0{\mydangersymbol{#1}\space}%
  \hangindent\wd0
  \parindent\hangindent
  \hangafter=-2
  \smash{\llap{\box0}}%
  \small
  \ignorespaces
 }{\par\smallskip}

\begin{document}

The symbol \mydangersymbol{A} is used to indicate an anecdote,
whereas the symbol \mydangersymbol{C} is used to indicate a caveat.
Note that using these symbols inline will cause horrible spacing.
\begin{mydanger}{A}
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
\end{mydanger}
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
\begin{mydanger}{C}
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.
Lorem ipsum dolor sit text to fill the line.

And a new paragraph within this \texttt{mydanger}
environment to show what happens when you have
more than one paragraph.
\end{mydanger}

\end{document}

Answer 2

参见教科书第44页：

另请参见此处：有没有简单的方法可以获得 Bourbaki 危险弯道符号（无需额外）？ 或者这里：https://en.wikipedia.org/wiki/Bourbaki_dangerous_bend_symbol。