how to get list of not displayed (hidden or unloaded) buffers

Hello
I want to know, how i can identify buffer, if it's displayed on screen
or not.

For example i open buffer 'option.c', then open buffer 'file2.txt' in
same window/tab.
'option.c' became hidden:

----
:buffers
25 #h "d:\workspace\sources\gvim_73\vim-7.3\vim73\src\option.c"
line 1
26 %a "file2.txt" line 1
Press ENTER or type command to continue
----

i need something like this:

for buffer_ in bufnr('$'))
if buffer_ == "not displayed"
do .....
endif
endfor

thanks in advance
Sergey V.

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