The -me macro package has an undocumented register, @, that the user can set to specify a debugging level.

This is probably only useful to -me developers, so it makes sense that this wouldn't be documented in the user-facing -me documents doc/meref.me.in and doc/meintro.me.in.  But it ought to be mentioned in developer-facing -me documentation--which at present really only consists of minimal comments in tmac/e.tmac itself.  (And it is, sort of--but buried in the middle of the file, and without explanation.)

Thus the attached patch adds a few lines of comments about the register, next to other general comments about the macro file at the top of the file.

Open questions:

• I think it's important to say that the register must be set before the package is loaded, but the instructions in this patch about how might be overexplaining to a developer.
• Someone more ambitious than me could figure out if each of the numbers 2 through 5 represents some specific category of debugging info that is output, and document that as well.
• If people think this capability is useful to ordinary -me users as well, then this should be placed in the user-facing documentation instead of in the source code.