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(gnus)Low-Level Threading

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Low-Level Threading
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`gnus-parse-headers-hook'
     Hook run before parsing any headers.

`gnus-alter-header-function'
     If non-`nil', this function will be called to allow alteration of
     article header structures.  The function is called with one
     parameter, the article header vector, which it may alter in any
     way.  For instance, if you have a mail-to-news gateway which
     alters the `Message-ID's in systematic ways (by adding prefixes
     and such), you can use this variable to un-scramble the
     `Message-ID's so that they are more meaningful.  Here's one
     example:

          (setq gnus-alter-header-function 'my-alter-message-id)
          
          (defun my-alter-message-id (header)
            (let ((id (mail-header-id header)))
              (when (string-match
                     "\\(<[^<>@]*\\)\\.?cygnus\\..*@\\([^<>@]*>\\)" id)
                (mail-header-set-id
                 (concat (match-string 1 id) "@" (match-string 2 id))
                 header))))