Gmail search trick

I am a heavy user of GMail. My account has tens of thousand emails categorized with about 40 or 50 labels.
I've been trying to figure out a way to select/list all mails labelled X and unread. After a long search I have tried the following search:

label:X and label:Unread

and to my surprise it worked as expected.
Update: Cedric pointed out in blog that was talking about the exact same topic that you may also use:

is:unread