Put in minimal simpset to avoid excessive simplification,
just as in revision 1.9 of HOL/indrule.ML
\documentclass[a4paper,11pt]{article}
\usepackage{latexsym,latexsym,amssymb,supertab}
\begin{document}
\begin{center}
{\Large
Keyboard Mapping for Function Keys F1 - F12}\\
Date: \today
\vspace*{.5cm}
\tablehead{\hline
Modifier & F1 &F2 &F3 &F4 &F5 &F6 &F7 &F8 &F9 & F10 &F11 &F12\\
\hline}
\tabletail{\hline}
\begin{supertabular}{|l|l|l|l|l|l|l|l|l|l|l|l|l|}
None& & & & & & & & &\mbox{$\bigwedge$}&\mbox{$\Longrightarrow$}&\mbox{$\equiv$}&\mbox{$\not=$}\nextline
\hline
Shift&\mbox{$\Leftarrow$}&\mbox{$=$}&\mbox{$\Rightarrow$}&\mbox{$\Leftrightarrow$}&\mbox{$\oplus$}&\mbox{$\ominus$}&\mbox{$\otimes$}&\mbox{$\oslash$}&\mbox{$\natural$}&\mbox{$\infty$}&\mbox{$\le$}&\mbox{$\ge$}\nextline
\hline
Alt&\mbox{$\leftarrow$}&\mbox{$-$}&\mbox{$\rightarrow$}&\mbox{$\leftrightarrow$}&\mbox{$\twoheadrightarrow$}&\mbox{$\mapsto$}&\mbox{$\leadsto$}&\mbox{$\uparrow$}&\mbox{$\downarrow$}&\mbox{$\emptyset$}&\mbox{$\in$}&\mbox{$\notin$}\nextline
\hline
AltGraph&\mbox{$\cap$}&\mbox{$\cup$}&\mbox{$\bigcap$}&\mbox{$\bigcup$}&\mbox{$\subseteq$}&\mbox{$\Box$}&\mbox{$\Diamond$}&\mbox{$\circ$}&\mbox{$\bullet$}&\mbox{$\parallel$}&\mbox{$\surd$}&\mbox{\copyright}\nextline
\hline
Ctrl&\mbox{$\sqcap$}&\mbox{$\sqcup$}&\mbox{$\overline{|\,\,|}$}&\mbox{$\bigsqcup$}&\mbox{$\lceil$}&\mbox{$\rceil$}&\mbox{$\lfloor$}&\mbox{$\rfloor$}&\mbox{$(\!|$}&\mbox{$|\!)$}&\mbox{$[\![$}&\mbox{$]\!]$}\\
\hline
Meta& & & & & & & & & & & & \nextline
\end{supertabular}
\end{center}
\end{document}