Integers: Sign extension
	We may need to change the number of bits used to represent a number.
	For example, C has int, short int, and long int to represent integers.
	The language doesn't specify the exact size of each form, but
		sizeof (short int) <= sizeof (int) <= sizeof (long int)
	How do we cast from one kind of int to a larger number of bits?
		ints are represented in N-bit 2C
		To convert a number from N bits to N + K bits in 2C notation,
			copy (extend) the sign bit from bit N-1 to bit N + K -1
		If the value is positive, all the higher-order bits will become 0;
			if the value is negative, all the higher-order bits will become 1.
		All of the other bits (location N-2, . . . , 0) remain unchanged.