Arithmetic: addition
	Unsigned binary addition follows the same rules as ordinary arithmetic
	Example: 4-bit UB
		carry	0	1	1	1
				0	0	1	1		3
				0	1	1	1		7
			result	1	0	1	0		10
	Two's complement addition also follows the same rules as UB
	Example: 4-bit 2C
		carry	1	1	1	1
				1	0	1	1		-5
				0	1	1	1		7
			result	0	0	1	0		2
		Note that the carry from the high-order bit may be ignored.
	How do we subtract?
		Negate the number to be subtracted and add
		a - b = a + (-b)