in=0000; co/sum = 0/0
in=0001; co/sum = 0/1
in=0010; co/sum = 0/1
in=0011; co/sum = 1/0
in=0100; co/sum = 0/1
in=0101; co/sum = 1/0
in=0110; co/sum = 1/0
in=0111; co/sum = 1/1