\newcommand 中的 \substack 可选参数获取 ! \\setof 的使用与其定义不符

Question

\documentclass{article}

\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{ifthen}
\usepackage{mathtools}

% Set former {x|P(x)} and {x,y,z}
\newcommand {\setof} [2] []
   { \ifthenelse {\equal{#1}{def}}
       {\{ #2 \}}
       {\{ #2 | #1 \} p1=#1}
   }

\begin{document}
Test setof substack = $\setof[\exists_{\substack{{x \\ y}}}] {foo}$

\end{document}

获取错误信息

! Use of \\setof doesn't match its definition.
\new@ifnextchar ...served@d = #1\def \reserved@a {
                                                  #2}\def \reserved@b {#3}\f...
l.17 ...setof[\exists_{\substack{{x \\ y}}}] {foo}

Answer 1

这个错误是典型的“移动参数中的脆弱命令”错误（谷歌搜索该短语:-)

在这种情况下：

\documentclass{article}

\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{ifthen}
\usepackage{mathtools}

% Set former {x|P(x)} and {x,y,z}
\newcommand {\setof}[2][]
   {\ifthenelse {\equal{#1}{def}}
       {\{ #2 \}}
      {\{ #2 | #1 \} p1=#1}%
   }

\begin{document}
Test setof substack = $\setof[\exists_{\protect\substack{x \\ y}}] {foo}$

\end{document}