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Format of Version 5 Babyl Files:

(Babyl was the storage format used by Rmail before Emacs 23.1.
Since then it uses mbox format.)

Warning:

    This was written Tuesday, 12 April 1983 (by Eugene Ciccarelli),
based on looking at a particular Babyl file and recalling various
issues. Therefore it is not guaranteed to be complete, but it is a
start, and I will try to point the reader to various Babyl functions
that will serve to clarify certain format questions.

    Also note that this file will not contain control-characters,
but instead have two-character sequences starting with Uparrow.
Unless otherwise stated, an Uparrow <character> is to be read as
Control-<character>, e.g. ^L is a Control-L.

Versions:

    First, note that each Babyl file contains in its BABYL OPTIONS
section the version for the Babyl file format. In principle, the
format can be changed in any way as long as we increment the format
version number; then programs can support both old and new formats.

    In practice, version 5 is the only format version used, and the
previous versions have been obsolete for so long that Emacs does not
support them.


Overall Babyl File Structure:

    A Babyl file consists of a BABYL OPTIONS section followed by
0 or more message sections. The BABYL OPTIONS section starts
with the line "BABYL OPTIONS:". Message sections start with
Control-Underscore Control-L Newline. Each section ends
with a Control-Underscore. (That is also the first character
of the starter for the next section, if any.) Thus, a three
message Babyl file looks like:

BABYL OPTIONS:
...the stuff within the Babyl Options section...
^_^L
...the stuff within the 1st message section...
^_^L
...the stuff within the 2nd message section...
^_^L
...the stuff within the last message section...
^_

    Babyl is tolerant about some whitespace at the end of the
file -- the file may end with the final ^_ or it may have some
whitespace, e.g. a newline, after it.


The BABYL OPTIONS Section:

    Each Babyl option is specified on one line (thus restricting
string values these options can currently have). Values are
either numbers or strings. The format is name, colon, and the
value, with whitespace after the colon ignored, e.g.:

Mail: ~/special-inbox

    Unrecognized options are ignored.

    Here are those options and the kind of values currently expected:

    MAIL Filename, the input mail file for this
Babyl file. You may also use several file names
separated by commas.
    Version Number. This should always be 5.
    Labels String, list of labels, separated by commas.


Message Sections:

    A message section contains one message and information
associated with it. The first line is the "status line", which
contains a bit (0 or 1 character) saying whether the message has
been reformed yet, and a list of the labels attached to this
message. Certain labels, called basic labels, are built into
Babyl in a fundamental way, and are separated in the status line
for convenience of operation. For example, consider the status
line:

1, answered,, zval, bug,

    The 1 means this message has been reformed. This message is
labeled "answered", "zval", and "bug". The first, "answered", is
a basic label, and the other two are user labels. The basic
labels come before the double-comma in the line. Each label is
preceded by ", " and followed by ",". (The last basic label is
in fact followed by ",,".) If this message had no labels at all,
it would look like:

1,,

    Or, if it had two basic labels, "answered" and "deleted", it
would look like:

1, answered, deleted,, zval, bug,

    The & Label Babyl Message knows which are the basic labels.
Currently they are: deleted, unseen, recent, and answered.

    After the status line comes the original header if any.
Following that is the EOOH line, which contains exactly the
characters "*** EOOH ***" (which stands for "end of original
header"). Note that the original header, if a network format
header, includes the trailing newline. And finally, following the
EOOH line is the visible message, header and text. For example,
here is a complete message section, starting with the message
starter, and ending with the terminator:

^_^L
1,, wordab, eccmacs,
Date: 11 May 1982 21:40-EDT
From: Eugene C. Ciccarelli <ECC at MIT-AI>
Subject: notes
To: ECC at MIT-AI

*** EOOH ***
Date: Tuesday, 11 May 1982 21:40-EDT
From: Eugene C. Ciccarelli <ECC>
To: ECC
Re: notes

Remember to pickup check at cashier's office, and deposit it
soon. Pay rent.
^_

;;; Babyl File BNF:

;;; Overall Babyl file structure:


Babyl-File ::= Babyl-Options-Section (Message-Section)*


;;; Babyl Options section:


Babyl-Options-Section
::= "BABYL OPTIONS:" newline (Babyl-Option)* Terminator

Babyl-Option ::= Option-Name ":" Horiz-Whitespace BOptValue newline

BOptValue ::= Number | 1-Line-String



;;; Message section:


Message-Section ::= Message-Starter Status-Line Orig-Header
EOOH-Line Message Terminator

Message-Starter ::= "^L" newline

Status-Line ::= Bit-Char "," (Basic-Label)* "," (User-Label)* newline

Basic-Label ::= Space BLabel-Name ","

User-Label ::= Space ULabel-Name ","

EOOH-Line ::= "*** EOOH ***" newline

Message ::= Visible-Header Message-Text


;;; Utilities:

Terminator ::= "^_"

Horiz-Whitespace
::= (Space | Tab)*

Bit-Char ::= "0" | "1"
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