How to include just some subdirectory excluding all envelop tree

I have some tree structure:

/a /a/a /a/b .. /a/z /b .. (thousands of folders) /z

How can I control just /a/a folder? What exclude file should be?

The way we use now is:

/a/* /b/ .. (thousands of folders) /z/ !/a/a

But it is very not nice looking and hard for maintenance...