backprop — bug → spec Plan-then-execute fixes the code & forgets. SDD fixes the code AND edits spec so recurrence is impossible. That edit is backprop. WHEN TO BACKPROP - Test failed at verification. - User reports bug. - Post-mortem after production incident. - flags VIOLATE with root cause found. SIX STEPS 1. TRACE Read failure output / bug report. Find exact file:line of wrong behavior. Name root cause in one caveman sentence. 2. ANALYZE Ask three questions: - Would a new §V invariant catch this class of bug? (most common: yes) - Is §I wrong — did spec claim shape the code cannot deliver?…