Binary Logic Question

I can''t seem to figure out the binary circuit for a 1-bit binary subtractor. I figured the adder out, and then confirmed it on google. I can''t figure out the subtractor, and I can''t find it on google. Adder truth table:

A   B   R   F

1   1   0    C
1   0   1   NC
0   1   1   NC
0   0   0   NC

Subtractor truth table:

A   B   R   F

1   1   0   NC
1   0   1   NC
0   1   1    C
0   0   0   NC

In the adder I could set the correct carry with a simple AND gate (1 and 1 = C). However, the subtractor confuses me. I can''t figure out the logic for: 0 ? 1 = C. I''ve tried lots of circuits, none of which work. Any help would be appreciated. Thanks.

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