how does one represent an alternating set of elements?

greetings;

i hope this is trivial for somebody, it's been an absolute hair puller for
me and some other people.

i am attempting to describe a set of alternating elements:

so, the valid collections of elements would be 

A, AB, ABA, ABAB,....

what would *not* be legal would be:

B, BA, AA, ABB, ABAA, BB,....

any help in describing this would be greatly appreciated!

tnx!

johnu