Compute the Kripke structure K={I′,S′,R′,L′}of the FSM: A={2Vin,2Vout,I,2Vstate,R} where Vin={a}, Vout={}, Vstate={p,q}, I=q, R=a&p&q&!next(q)&next(p)|a&q&!p&!next(p)&next(q)|a&!q&!next(p)&next(q)

I am asked to encode the set of reachable states S′reach. This is what I got

!p&q&!a | !p&q&a | p&!q&!a | p&!q&a | p&q&!a | p&q&a

but it is marked as incorrect, can you please tell me why?