Create a woodpecker script that takes 16-bit a and 16-bit b and calculates the 16-bit difference a-b.
If b>a, output its two's complement.
<low a hi> | <low b hi> | <low output hi>
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0000000000000000 | 0000000000000000 | 0000000000000000
0100000000000000 | 1000000000000000 | 1000000000000000
0000000000000000 | 1000000000000000 | 1111111111111111
0000100000000000 | 1111000000000000 | 1000000000000000
0010010110000000 | 1010001000000000 | 1111101010000000
1010001000000000 | 0010010110000000 | 1000010101111111
1111111111111111 | 1111111111111111 | 0000000000000000