Activity which fires when a wfd variable changes state

I am asking this question again in hopes somebody will take a few minutes 
to answer:

I have an activity which has to fire only when the value of a wfd variable 
changes (in a previous activity). First, can this information be coded 
into the xpdl? If so, how? If not, how do people attack this kind of 
situation? If there is documentation on this topic, a link would be 
great.. 

I would be very very grateful for any help.
thanks in advance,
Geeta

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