Local stability is scanned across exact slope magnitudes. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A small slope is an accepted local contract
The first checked source has slope one half at the fixed point.
∣m∣=21<1
The threshold uses magnitude, not sign
The neutral reflection row has slope negative one. Its magnitude is one, so it is not accepted by the less-than-one contract.