If L1 and L2 are languages, the difference of L1 and L2 is L1 - L2 = {x | x L1 and x L2}.

a. Using the product construction to combine two DFAs M1 = (Q, ,1 , q0, F1) and M2 = (R, , 2, r0, F2), what should the set of accepting states be so that the resulting machine accepts L(M1) - L(M2)?

b. Prove that the regular sets are closed for set difference, without using the product construction