The circuit description, as given in the first log entry, does not provide subtraction. 

This was added by simply putting four exclusive-or-gates in the lines for the A inputs. 

The function input F0 is now used to control these exor gates, and is no longer connected to the multiplexers. These inputs of the multiplexer are connected to logic zero. They were used for negating the B input, but negating an input can now be done with the new exor gates.