Counter
	Consider what it means to count in binary:
			x2	x1	x0	value
			0	0	0	0
			0	0	1	1
			0	1	0	2
			0	1	1	3
			1	0	0	4
			1	0	1	5
			1	1	0	6
			1	1	1	7
	The sequence of x0 values looks like a clock with period 1: 0 1 0 1 0 1 0 1
	The sequence of x1 values looks like a clock with period 2: 0 0 1 1 0 0 1 1
	The sequence of x2 values looks like a clock with period 4: 0 0 0 0 1 1 1 1
	However, notice when x1 changes relative to x0:
		x1 goes from 0 to 1 (for example, value 1 to 2) when x0 goes from 1 to 0
		x1 goes from 1 to 0 (for example, value 3 to 4) when x0 goes from 1 to 0
	This means that we need to toggle x1 when x0 is on a negative edge,
		but we want to use positive-edge flip-flops.