bachotex2009-hans-taco-pearl3.tex
bachotex2009-hans-taco-pearl3.tex
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TeX document,
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%%% $$: empty formula or unmatched display?
% When you try the following under \TeX's normal catcode
% regime, you will get an error:
$$ $21-09$
% The message is:
% ! Display math should end with $$.
% <to be read again>
% 2
% l.7 $$ $2
% 1-09$
% But how about this then:
\halign{#&#\cr $$ & $21-09$\cr}
% It magically works! The actual effect is similar to
\hbox{$$} % or
\hbox{${}$}
% In words of the \TeX{}book (chapter 25, page 287):
%
% One consequence of these rules is that `$$' in restricted
% horizontal mode simply yields an empty math formula.
\end
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