{"id":"digital-root-zero-special-case","text":"The digital root formula `1 + (n-1) % 9` requires an explicit `num == 0` guard because `(-1) % 9 == 8` in Python, which would return 9 instead of 0.","truth_value":"IN","source":"entries/2026/06/06/add-digits-solution.md","source_url":"","source_hash":"","justifications":[],"dependents":[],"metadata":{},"created_at":"","updated_at":"","reviewed_at":"","verified_at":"","retracted_at":"","explanation":{"steps":[{"node":"digital-root-zero-special-case","truth_value":"IN","reason":"premise"}]}}