A lineage is a descent group: under patrilineal descent a child joins its father's line, under matrilineal its mother's, under bilateral descent no bounded line survives — membership drifts, and small lines are absorbed by large ones.
Most societies forbid marriage inside your own lineage (exogamy). Claude Lévi-Strauss saw that this taboo is not a prohibition but an engine: it forces every lineage to obtain its spouses from another, turning marriage into exchange. A rule of cross-cousin marriage prescribes exactly whom — so the same lines swap partners generation after generation, braiding rivals into a permanent alliance web.
Drop that rule for free choice and the exchange scatters: alliances never repeat, lineages drift, and the village slowly fragments. Kinship diagrams look dry — but descent and alliance are a generative system, and you are running it.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.