David Carlisle <d.p.carlisle at gmail.com> writes:
> \input picture
> \catcode`\@=11
>
> \RequirePackage{cancel}
> \let\zzcancel\cancel
> \def\cancel#1{{%
> \let\line\LaTeX at line
> \zzcancel{#1}}}
>
> $\cancel{x^2 + 1}$
>
> $\cancel{a}$
>
> $\cancel{{x^2 + 1 \over b}}$
>
>
> \bye
Fantastic...!!