Define rules for AllegroGraph triples and how to apply them

Question

I'm using AllegroGraph to store statement like this:

<newsid1  hasAnnotation  Gamma>
<newsid1  hasAnnotation Beta>

I would like to define a rule on this staments that says: if the subject newsid1 hasAnnotation either Gamma or Beta, then add a new statement in the triplestore that says that the subject hasAnnotation Theta, i.e. the statement

<newsid1  hasAnnotation Theta>

My questions are the following:

1. How I can define such a rule for Allegro?
2. How can I apply these rules over the statements?