help needed with gnus mode-line

[Thierry Volpiatto <thierry.volpiatto@gmail.com>]

Hi all,
i would like to display temporarily information in mode-line while i am
in gnus.
In all emacs, something like this work fine:

--8<---------------cut here---------------start------------->8---
(let ((mode-line-format (concat " " str)))
  (force-mode-line-update)
  (sit-for 12))
(force-mode-line-update))
--8<---------------cut here---------------end--------------->8---

But in gnus the mode-line isn't modified.

I saw gnus have it's own mode-line-format for each mode, but i am unable
to use it.

Help is welcome.

Thanks.

-- 
A+ Thierry
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