Write VHDL code for the circuit corresponding to textbook problem 4.40 using structural VHDL (port maps).

4.40) A half adder is a circuit that adds two bits to give a sum and a carry. Give the truth table for a half adder, and design the circuit using only two gates. Then design a circuit which will find the 2's complement of a 4-bit binary number. Use four half adders and any additional gates. (Hint: Recall that one way to find the 2's complement of a binary number is to complement all bits, and then add 11.)