Brain Teaser!!

I have two example tables R,S as seen above, I want to write a query that returns all attributes A (left table) that are associated with exactly 'b2' and 'b4' from the other table, in this case it should return only a2.

Using division returns both a1 and a2 since they both have that association, but a1 does not apply since it has association with other attributes, only allowed to use minimal relational algebra operators