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Appendix B — The Software Level

Not a finer re-run of the system tree — the interfaces it cannot see.

A clean system tree already hands the software its requirements, so a software fault tree is not a finer-grained re-run of it. It earns its place only for the internal interfaces the system level cannot express: shared dependencies that turn independent-looking elements into common causes, and interference across shared memory. The wiper example shows both.