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\(ewcommand{\N}{\mathbb N} ewcommand{\Z}{\mathbb Z} ewcommand{\Q}{\mathbb Q} ewcommand{\R}{\mathbb R}
ewcommand{\lt}{<}
ewcommand{\gt}{>}
ewcommand{\amp}{&}
\definecolor{fillinmathshade}{gray}{0.9}
ewcommand{\fillinmath}[1]{\mathchoice{\colorbox{fillinmathshade}{$\displaystyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\textstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptstyle \phantom{\,#1\,}$}}{\colorbox{fillinmathshade}{$\scriptscriptstyle\phantom{\,#1\,}$}}}
\)
