Subtraction is truth preserving on the sign bit. It's not truth-preserving in the actual subtractive bits.
(I disagree with their claim that the subtractive bit is functionally complete on its own - you're right, since it's truth-preserving, it clearly is not functionally complete)
(I disagree with their claim that the subtractive bit is functionally complete on its own - you're right, since it's truth-preserving, it clearly is not functionally complete)