5.5.31 ‘@z’: Begin ignorable material, or terminate change

FWEB files may begin with the construction

@z
.
.
@x

where the ‘@z’ occupies the very first two characters of the file, and where the ‘@z’ and ‘@x’ must begin in column 1. Material between the ‘@z’ and ‘@x’ is pure commentary and is ignored by both processors, with one exception.

The exception is that an RCS-like line (RCS stands for “revision-control system”) with syntax

$Keyword: Text of Keyword $

(at least one blank after the colon, and at least one before the last dollar sign; UNIX users, see ‘man ident’) is parsed, and the text of the Keyword is made available to the control codes ‘@K’ (see ATK_) and ‘@k’ (see ATk) as well as to FTANGLE’s built-in function $KEYWORD (see $KEYWORD.

A distinction is made between keywords that are found in the ignorable commentary at the beginning of the master web file, which are called global keywords, and ones that are found at the beginning of files included via ‘@i’, which are called local keywords.

The commands that access RCS-like keywords function as follows:

• ‘$KEYWORD(Keyword)’ accesses a global keyword. It is a built-in function that is expanded by FTANGLE (during output) into the quoted character string "Text of Keyword".
• ‘@K’ and ‘@k’ are expanded during input. ‘@K’ accesses a global keyword, whereas ‘@k’ accesses a local keyword.
• In the limbo section or a TeX part, FWEAVE will expand ‘@K Keyword @>’ into Text of Keyword (without the surrounding quotes), and similarly for ‘@k’. (The intention is that the expanded text can be used as bodies of TeX macros.) FWEAVE will also print the values of global keywords at the end of its output, whether or not they are referenced by ‘@K’.
• Elsewhere FWEAVE will just print the keyword name itself, surrounded by double angle brackets. If the keyword was local (‘@k’), the brackets will carry the subscript 0.
• FTANGLE treats the global command ‘@K Keyword @>’ essentially like it does ‘$Keyword’, except that the construction is expanded on input rather than output.
• FTANGLE expands the command ‘@k keyword @>’ on input, generating a quoted string containing the value of the local keyword.

The command ‘@z’ is also used in change files to end a change. See Change files.