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3.9.1.4 Low-Level Threading
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-IDs in systematic ways (by adding prefixes and such), you can use this variable to un-scramble the Message-IDs 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))))