# Why $$and$$ were not defined as robust commands in the first place?

I am aware of the difference between using $$...$$ and $...$ (in moving arguments, for example), and yes, I've read the answers to this question: Are $$and$$ preferable to $?). I also know the definition of $$ and $$:  \def$${\relax\ifmmode\@badmath\else\fi} \def$${\relax\ifmmode\ifinner$\else\@badmath\fi\else\@badmath\fi}


(taken from Section 55.2 of source2e.pdf)

My question here is: couldn't $$ and $$ have been defined as robust commands in the LaTeX kernel? Is there any (non trivial) reason explaining why they were defined as fragile commands?

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Since the fixltx2e package ends with

\MakeRobust$$\MakeRobust$$
\MakeRobust$\MakeRobust$
\MakeRobust\makebox
\MakeRobust\savebox
\MakeRobust\framebox
\MakeRobust\parbox
\MakeRobust\rule
\MakeRobust\raisebox


it is probably safe to say that it was simply a mistake that was left around because the kernel can't change at this point. It's also worth pointing out that this is covered in Section 4 in the fixltx2e documentation titled “Fixes added for 2005/12/01.”

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