%%% $$: 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
