#decl443#
In LATEX~2.09, if you wanted a command to work both in math mode and
in text mode, the suggested method was to define something like:
verbatim53#
Unfortunately, the || stops || changing size correctly in (for
instance) subscripts or a fraction.
In LATEX2e<#448#><#448#> you can define it thus:
verbatim54#
Now || will work correctly in all contexts.
This is because the || does nothing, producing simply
|G_p|, when || is used within math mode; but it ensures that math
mode is entered (and exited) as required when || is used in text
mode.