Editing Metasyntactic variable (section)

===IETF Requests for Comments===
Both the [[IETF]] [[Request for Comments|RFC]]s and [[computer programming language]]s are rendered in [[plain text]], making it necessary to distinguish metasyntactic variables by a naming convention, since it would not be obvious from context.

Here is an example from the official [[IETF]] document explaining the [[e-mail]] protocols (from RFC 772 - cited in RFC 3092):
  All is well; now the recipients can be specified.
 
      S: MRCP TO:<Foo@Y> <CRLF>
      R: 200 OK
 
      S: MRCP TO:<Raboof@Y> <CRLF>
      R: 553  No such user here
 
      S: MRCP TO:<bar@Y> <CRLF>
      R: 200 OK
 
      S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
      R: 200 OK
 
   Note that the failure of "Raboof" has no effect on the storage of
   mail for "Foo", "bar" or the mail to be forwarded to "fubar@Z"
   through host "X".

(The documentation for texinfo emphasizes the distinction between metavariables and mere variables used in a programming language being documented in some texinfo file as: "Use the @var command to indicate metasyntactic variables. A metasyntactic variable is something that stands for another piece of text. For example, you should use a metasyntactic variable in the documentation of a function to describe the arguments that are passed to that function. Do not use @var for the names of particular [[Variable (programming)|variables]] in programming languages. These are specific names from a program, so @code is correct for them."<ref>{{cite web |url=http://sunsite.ualberta.ca/Documentation/Gnu/texinfo-4.0/html_chapter/texinfo_10.html |title=Marking Words and Phrases |work=Texinfo 4.0. The GNU Documentation Format|access-date=2012-11-25 |url-status=dead |archive-url=https://web.archive.org/web/20091106004856/http://sunsite.ualberta.ca/Documentation/Gnu/texinfo-4.0/html_chapter/texinfo_10.html |archive-date=2009-11-06 }}</ref>)

Another point reflected in the above example is the convention that a metavariable is to be uniformly substituted with the same instance in all its appearances in a given schema. This is in contrast with [[nonterminal]] symbols in [[formal grammars]] where the nonterminals on the right of a production can be substituted by different instances.<ref name="Tennent2002">{{cite book|author=R. D. Tennent|title=Specifying Software: A Hands-On Introduction|year=2002|publisher=Cambridge University Press|isbn=978-0-521-00401-5|pages=36–37 and 210}}</ref>