Add an \LTextract macro

[torik42]

It would be great to have a macro like

Macro(parms, '\\LTextract', args='A', extract='#1')

which could be used to check parts of custom commands separately.
The user would maybe define

\newcommand{\LTextract}[1]{#1}

in the preamble.

The name, of course, is only a suggestion, with the YY naming scheme maybe \YYExtract.

I could try to add it.