I am making notes with blanks to fill in and would like to produce an instructor version with blanks filled in and a student version with blanks unfilled.
I would like a \fillin{ . . . } command that wraps text and allows math. The solution from Create fill-in-the-blank version of a document with ability to toggle blanks on and off works well for wrapping text, but censor does not allow math (or other font switching):
\documentclass{article}
\usepackage{censor}
\censorruledepth=-.2ex
\censorruleheight=.1ex
\begin{document}
\xblackout{this works fine this works fine this works fine this works fine this works fine this works fine this works fine this works fine this works fine this works fine }
\xblackout{this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$ this does not work $a+b$}
\end{document}
One idea I tried was to use ulem's uline, but color text white for the student version. This works for math mode, but text-wrapping fails:
\documentclass{article}
\usepackage{ulem,xcolor}
\newcommand{\fillin}[1]{\uline{\textcolor{white}{#1}}}
\begin{document}
\fillin{This allows math mode $a+b=c$, but the text-wrapping is broken This allows math mode $a+b=c$, but the text-wrapping is broken This allows math mode $a+b=c$, but the text-wrapping is broken}
\end{document}

\newcommand{\soulfill}[1]{\color{red}{\ul{#1}}\color{black}}in order to change the text back to black after the underlining is done. – Chris Chudzicki Nov 27 '13 at 14:53\textcolor, not\color. So\newcommand{\soulfill}[1]{\textcolor{red}{\ul{#1}}}should confine the coloring to just the underlined texts. – Herr K. Nov 27 '13 at 16:58