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...