Why does using the amsmath package break the following code?

Question

I am trying to find a way to hide expressions inside a math environment. I found a solution, provided by Steven B. Segletes at

Hiding part of text leaving blank space

However, when I add the amsmath package, it breaks, giving me the error "Missing $ inserted". I have provided a MWE example below. If I remove \usepackage{amsmath}, it compiles. If I remove the \frac command it compiles.

\documentclass{article}

\usepackage{amsmath}

\usepackage{censor}
\usepackage{stackengine}
\usepackage{scalerel}

\censorruleheight=0ex

\makeatletter
\long\def\blackout#1{%
 \def~{-}%
 \protected@edef\save@arg{#1}%
 \expandafter\censor@Block\save@arg\stringend\let~\sv@tilde}
 \let\sv@cenword\@cenword
\newcommand\m@cenword[1]{\ThisStyle{%
\stackengine{\mcensorruledepth}{$\SavedStyle\phantom{#1}$}%
{\rule{\widthof{$\SavedStyle#1$}}{\the\censorruleheight}}{U}{c}{F}{T}{L}}}
\newcommand\mblackout[2][\dp\strutbox]{%
 \let\@cenword\m@cenword%
\def\mcensorruledepth{#1}%
\blackout{{#2}}%
\let\@cenword\sv@cenword%
}
\makeatother

\begin{document}

\[\mblackout{\frac{a}{b}} \]

\end{document}

Answer 1

With version 4.2 of the censor package, the censoring of math text is handled somewhat automatically. For example:

\documentclass{article}
\usepackage{amsmath}
\usepackage{censor}
%\censorruleheight=0ex
\begin{document}
\[y = \blackout{x = \frac{a_{1}}{b_{2}}} \cdot \frac{1}{2} \]

\[y = x = \frac{a_1}{b_2} \cdot \frac{1}{2} \]

$\beta / \blackout{\alpha} \times 3$

$\beta / \alpha \times 3$
\end{document}

yields

Note several points: 1) non-text macros like \frac are not censored, per se, but only the numerator and denominator text are censored (thus the vinculum shows), and 2) censored sub- and superscripts must be embraced even when a single token.

Therefore, for the current question, where the \censorruleheight can be set to 0pt, the vinculum remains:

ORIGINAL ANSWER

You can set the offending macros in their own group (via braces) and then they set just fine, with or without amsmath:

\documentclass{article}

\usepackage{amsmath}

\usepackage{censor}
\usepackage{stackengine}
\usepackage{scalerel}

\censorruleheight=0ex

\makeatletter
\long\def\blackout#1{%
 \def~{-}%
 \protected@edef\save@arg{#1}%
 \expandafter\censor@Block\save@arg\stringend\let~\sv@tilde}
 \let\sv@cenword\@cenword
\newcommand\m@cenword[1]{\ThisStyle{%
\stackengine{\mcensorruledepth}{$\SavedStyle\phantom{#1}$}%
{\rule{\widthof{$\SavedStyle#1$}}{\the\censorruleheight}}{U}{c}{F}{T}{L}}}
\newcommand\mblackout[2][\dp\strutbox]{%
 \let\@cenword\m@cenword%
\def\mcensorruledepth{#1}%
\blackout{{#2}}%
\let\@cenword\sv@cenword%
}
\makeatother

\begin{document}
\[y = \mblackout{x = {\frac{a}{b}}} \cdot \frac{1}{2} \]

\[y = x = \frac{a}{b} \cdot \frac{1}{2} \]

$\beta / \mblackout{{\alpha}} \times 3$

$\beta / \alpha \times 3$
\end{document}

Answer 2

why don't you just use \phantom{...}? \phantom never puts anything but (precisely measured) space on a page.

of course, you'll have to be careful about keeping things like & and other formatting instructions in proper synch, but it should be reasonably obvious how to do that.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[ abc\phantom{def}ghi \]

first, the example without \verb+\phantom+s:
\begin{align*}
 abc &= def\\
 ghi &= \frac{1}{m}jklmnopq\\
 rst &> \frac{1}{2}

\end{align*}
this even works in $abc\tfrac{1}{1+n}de$ in-line text.

now, apply \verb+\phantom+ to selected elements:
\begin{align*}
 abc &= def\\
 g\phantom{h}i &= \phantom{\frac{1}{m}}jk\phantom{lmn}opq\\
 rs\phantom{t} &> \frac{1}{2}
\end{align*}
this even works in $ab\phantom{c\tfrac{1}{1+n}}de$ in-line text.

\end{document}