[johnday]

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)

[gliptic]

The sign bit is the only bit involved here. What "subtractive bits" are you referring to?