6.9 Two ways for implementing a serial adder (A + B) are shown in Section 6.2. It is necessary to modify the circuits to convert them to serial subtractors (A - B). (a) Using the circuit of Fig. 6.5, show the changes needed to perform A + 2's comple- ment of B. (HDL-see Problem 6.35(h)) (b) Using the circuit of Fig. 6.6, show the changes needed by modifying Table 6.2 from an adder to a subtractor circuit. (See Problem 4.12.) (HDL-see Problem 6.35(i))