Is this constraint expressible in XML Schema?

Hi,

    I want to express the following constraint using XML schema :

    An unbounded sequence of (A or B) elements which can contain at
most n instances of A.

    To illiustrate, I set n = 2. In that case, BABBBBBABBB is valid.
So is AABB .  BABABA is not valid because there are more than 2 A
elements in it.

Thanks,
Kenneth