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(gnus)Mail-like Back Ends

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Mail-like Back Ends
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   One of the things that separate the mail back ends from the rest of
the back ends is the heavy dependence by the mail back ends on common
functions in `nnmail.el'.  For instance, here's the definition of
`nnml-request-scan':

     (deffoo nnml-request-scan (&optional group server)
       (setq nnml-article-file-alist nil)
       (nnmail-get-new-mail 'nnml 'nnml-save-nov nnml-directory group))

   It simply calls `nnmail-get-new-mail' with a few parameters, and
`nnmail' takes care of all the moving and splitting of the mail.

   This function takes four parameters.

METHOD
     This should be a symbol to designate which back end is responsible
     for the call.

EXIT-FUNCTION
     This function should be called after the splitting has been
     performed.

TEMP-DIRECTORY
     Where the temporary files should be stored.

GROUP
     This optional argument should be a group name if the splitting is
     to be performed for one group only.

   `nnmail-get-new-mail' will call BACK-END`-save-mail' to save each
article.  BACK-END`-active-number' will be called to find the article
number assigned to this article.

   The function also uses the following variables:
BACK-END`-get-new-mail' (to see whether to get new mail for this back
end); and BACK-END`-group-alist' and BACK-END`-active-file' to generate
the new active file.  BACK-END`-group-alist' should be a group-active
alist, like this:

     (("a-group" (1 . 10))
      ("some-group" (34 . 39)))