bug#2222: 23.0.90; Labels in RMAIL

[Richard M Stallman <rms@gnu.org>]

    If it is correctly documented and the message is well identified,
    that would not hurt to have it visible. A message with a subject
    like "Special rmail message, do not delete" could be enough.

If Rmail adds this special message
only when the file has messages with labels,
and if the subject says it exists to record the labels,
I think it will be ok to show these messages.
This way, most users won't have these messages, and those who do
will recognize them as serving a specific useful purpose.

The subject could be "Rmail label list for completion".

To hide these messages poses a number of problems that each need
to be solved.  It is less reliable.

For instance, what happens if you concatenate two mbox files to make a
longer one, and the second one has one of these magic messages at the
front?  It will end up in the middle of the combined file.  If these
messages are normally visible, the user will understand how it got
there.  If they are normally hidden, this one will appear out of the
blue.  Or should Rmail be prepared to hide any number of such
messages?  Should it automatically delete any that appear other than
at the start of the file?

Another advantage of showing them is that you can delete them if you
wish.  There might be occasions when you don't want any magic added
message in your file.  If you delete it, it will stay deleted until
you read the file into Rmail once again, or use commands to manage
labels.