Tagged text

The proposal

(as suggested text for cvsclient.texi)

@item MT @var{tagname} @var{data} \n

This response provides for tagged text.  It is similar to
SGML/HTML/XML in that the data is structured and a naive application
can also make some sense of it without understanding the structure.
The syntax is not SGML-like, however, in order to fit into the CVS
protocol better and (more importantly) to make it easier to parse,
especially in a language like perl or awk.

The @var{tagname} can have several forms.  If it starts with @samp{a}
to @samp{z} or @samp{A} to @samp{Z}, then it represents tagged text.
If the implementation recognizes @var{tagname}, then it may interpret
@var{data} in some particular fashion.  If the implementation does not
recognize @var{tagname}, then it should simply treat @var{data} as
text to be sent to the user (similar to an @samp{M} response).  There
are two tags which are general purpose.  The @samp{text} tag is
similar to an unrecognized tag in that it provides text which will
ordinarily be sent to the user.  The @samp{newline} tag is used
without @var{data} and indicates that a newline will ordinarily be
sent to the user (there is no provision for embedding newlines in the
@var{data} of other tagged text responses).

If @var{tagname} starts with @samp{+} it indicates a start tag and if
it starts with @samp{-} it indicates an end tag.  The remainder of
@var{tagname} should be the same for matching start and end tags, and
tags should be nested (for example one could have tags in the
following order @code{+bold} @code{+italic} @code{text} @code{-italic}
@code{-bold} but not @code{+bold} @code{+italic} @code{text}
@code{-bold} @code{-italic}).  A particular start and end tag may be
documented to constrain the tagged text responses which are valid
between them.

Note that if @var{data} is present there will always be exactly one
space between @var{tagname} and @var{data}; if there is more than one
space, then the spaces beyond the first are part of @var{data}.

Here is an example of some tagged text responses.  Note that there is
a trailing space after @samp{Checking in} and @samp{initial revision:}
and there are two trailing spaces after @samp{<--}.  Such trailing
spaces are, of course, part of @var{data}.

@example
MT +checking-in
MT text Checking in 
MT fname gz.tst
MT text ;
MT newline
MT rcsfile /home/kingdon/zwork/cvsroot/foo/gz.tst,v
MT text   <--  
MT fname gz.tst
MT newline
MT text initial revision: 
MT init-rev 1.1
MT newline
MT text done
MT newline
MT -checking-in
@end example

If the client does not support the @samp{MT} response, the same
responses might be sent as:

@example
M Checking in gz.tst;
M /home/kingdon/zwork/cvsroot/foo/gz.tst,v  <--  gz.tst
M initial revision: 1.1
M done
@end example

Discussion

• Do we want to be able to negotiate for tag names as for responses? Probably we won't need to very often, so perhaps analogues to @samp{UseUnchanged} would suffice.