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The Standard Fault Tree Pattern

One recurring step, applied along the signal path, grows the whole tree.

An output-signal failure has only two sources: the system processed correct inputs wrongly, or it passed through an already-faulty input — a system failure OR an input-signal failure. An input signal is some other system's output, so every leaf is again a function failure, the top of a fresh application of the same pattern. The tree grows in two directions at once, downward and backward.