I am having trouble figuring out how to do this in PB with an edir-edir

TreeA(subscriber): Attribute ATTR1(multivalue) can have 3 values A,B,or C or
any combination.
TreeB(publisher): syncs and received ATTR1

Now when any change comes in on ATTR1---lets say changing from A to B, or
from AB to A, or from B to AB, etc. What I need to happen is fire off an
action on the publisher if ATTR1 is *only* left with A.

I am having difficulties using the changing from/to. I have tried some
things querying dest-attr on the publisher, etc. But I cant wrap myself on
the logic. Someone throw me some hints. Thanks!