# M01-L05 — Lab contract

**Build:** Construct two forks with unequal cumulative work and simulate branch switch.

**Pass:** Heavier valid branch wins; orphaned transactions become unconfirmed.

**Failure injection:** Counting confirmations as irreversible leads to incorrect payment finality.

**Review question:** How does probabilistic finality differ from PoS finality?

Status: specification; only count as executable when source and tests are present and verified.