What is the :eval form ?

[Philippe M. Coatmeur <philippe.coatmeur@gmail.com>]

Hi ; I'm trying to get some information about this (e?)lisp form

  (add-to-list 'global-mode-string
  	       '(:eval (mail-bug-mode-line-all "2")))

But it appears to be really hard to google for it, as both

https://www.google.com/search?q=":eval"
https://www.google.com/search?q="eval"

Return the same results... (apparently google does not care for
punctuation)

How can I find information about this special form ? When I ask emacs
using C-h f with point over it, it acts like google and shows me the
"plain eval" info page... :(

BTW what I'm trying to do is to use a variable to pass the ("2" in the
example) argument to the function, to use it in a loop, like this :

  (loop for i from 1 to 3 do
	(add-to-list 'global-mode-string
		     '(:eval (mail-bug-mode-line-all (format "%s" i)))))

but i's value is always stuck at 1 :(

Phil