13. (8 pts) Develop a synchronous 2-bit up/down counter with the Grey code sequence using D flip-flops. The counter should count up, i.e. 00, 01, 11, 10, 00, ..., when the UP/DOWN' control input X is 1 and count down, i.e. 00, 10, 11, 01.00,..., when the control input is 0. (Notice that you need to draw the state diagram, next state table, flip-flop transition table, Karnaugh maps, Boolean expressions and counter imple. mentation)