\let\implies\Rightarrow
\let\epsilon\varepsilon
+%% A table heading cell. Clone and hack \multicolumn.
+\def\thd{\omit\@ifnextchar[\thd@{\thd@[l]}}
+\def\thd@[#1]#2{%
+ \begingroup
+ \tab@multicol \tab@initread \let\tab@looped\tab@err@multi
+ \tab@preamble{}\def\tab@midtext{\bfseries#2}\tab@readpreamble{#1}%
+ \the\tab@preamble
+ \endgroup \ignorespaces
+}
+
%% Unix manpage references.
\def\man#1#2{\textbf{#1}(#2)}