filter folders

[Andrea Crotti]

I'm using offlineimap 6.5.3.1 and I don't find a way to filter out the 
"All Mail" folder,
I have this setting:

folderfilter = lambda foldername: foldername not in ['Trash', 'Deleted 
messages', "[Gmail].*"]

 From the wiki page I found this:
folderfilter = lambda foldername: foldername not in '[Gmail]/All Mail'

but that in standard python would be wrong, because it since also a 
string is iterable it would
check that foldername is not a character contained in that string.

So how is exactly the semantic of filtering in the configuration file?

Thanks,
Andrea