add     swap            r!=0 && 15      15-r    result
0       0               0               0       a-b
0       0               0               1       15-(a-b), 15+b-a

0       0               1               0       (a>b) && 15
0       0               1               1       (a<=b) && 15

0       1               0               0       b-a
0       1               0               1       15-(b-a), 15+a-b

0       1               1               0       (a>b) && 15
0       1               1               1       (a>=b) && 15

        short
1       0               0               0       a+b
1       0               0               1       15-(a+b), 15-a-b

1       0               1               0       (a||b) && 15
1       0               1               1       (!a && !b) && 15

1       1               0               0       max(a,b)
1       1               0               1       15-max(a,b), min(15-a,15-b)

1       1               1               0       (b||a) && 15
1       1               1               1       (!b && !a) && 15

The first bit of the instruction controls whether to add or subtract. The
second bit controls whether to short/swap the two inputs. The third bit, if
active, takes the sum/difference produced by the adder/subtractor and sets
output to full power if it isn't zero. The fourth bit activates the analog
inverter.

When in subtract mode, the "swap" bit will swap inputs A and B. When in add
mode, the "swap" bit will actually short inputs A and B together, meaning that
both A and B will be equal to max(A,B).