How to realize binary adding and subtracting of polygons dropped by the Blitter onto playfield, by setting up it's minterms.

Adding is binary function of two sources and bit carried on from the previous binary point to another.
In case of blit we use channel A as a background, B as an object and C as a value of blit carried from previous bitplane - the overflow.

According to Basics of Digital Technology by Andrzej Skorupski, binary function of adding two operands goes like this:

A B C  D  O
-----------
0 0 0  0  0
0 0 1  1  0
0 1 0  1  0
0 1 1  0  1
1 0 0  1  0
1 0 1  0  1
1 1 0  0  1
1 1 1  1  1

interpreting this function as usual Karnaugh map we're having two blits - D and O. Because D operand overwrites operand A, blit O has
to be performed first. Writing minterms for blit D gives result %10010110 - $96. Blit O is %11101000 - $e8

The same book brings binary function for subtracting:

A B C  D  O
-----------
0 0 0  0  0
0 0 1  1  1
0 1 0  1  1
0 1 1  0  1
1 0 0  1  0
1 0 1  0  0
1 1 0  0  0
1 1 1  1  1

And this gives blit D - $96, like before and blit O (overflow, which is now underflow, loan or borrow) - $8e

For usual shade bobs it is casual that overflow or loan from the last position is carried on to the first bitplane.

To get saturation effect funstion for last bitplane has to be changed from adding to binary OR - $fe, for blit D and unconditional 0 for
blit O. For subtraction minterms $90 for blit D and $80 for blit O, should do the saturation right.
