Sorry about this.  A number of people had difficulty.  I must
admit I was a bit sloppy.  Let's try again.

> prove x-y = comp( comp(x) + y)
>

x-y = -(comp(x) + y) - 1    apply definition of 2's complement: -x = comp(x)+1
                            to comp(comp(x)+y)

    = -(-x - 1  + y) - 1    apply same definition to comp(x)

    = x - y

        DONE!



> Could you write the steps down for me
> (with explanation). I can't quite follow
> the proof.