Section 3.3.6 Choosing New Legislators
As old families emigrated and new ones immigrated, this system had to change. The Paxons decided to add and remove members of Parliament by decree. This posed a circularity problem: membership in Parliament was determined by which decrees were passed, but passing a decree required knowing what constituted a majority set, which in turn depended upon who was a member of Parliament. The circularity was broken by letting the membership of Parliament used in passing decree n be specified by the law as of decree n − 3. A president could not try to pass decree 3255 until he knew all decrees through decree 3252. In practice, after passing the decreeLAMPORT, P. 22 — §3.3.6
3252: Στρωνγ is now a legislator
the president would immediately pass the "olive-day" decree as decrees 3253 and 3254.
In modern parlance this is a configuration change which has entire papers written with theories and algorithms of its' own. This type is configuration change with a fixed lag. Here it's a two slot lag to allow catch-up and consensus before the new quorum is used
This is why Lamport emphasizes what constitutes a majority set: the quorum rules depend on the active configuration, and that configuration must be globally agreed upon before it can safely be used. The number of legislators might have changed and that should be known and set throughout the system and not just for the president.
Without careful preparation it is possible to livelock the service into a state that is impossible to recover from automatically. Below he describes a classic configuration deadlock: a valid quorum no longer exists, so no further progress is possible.
Changing the composition of Parliament in this way was dangerous and had to be done with care. The consistency and progress conditions would always hold. However, the progress condition guaranteed progress only if a majority set was in the Chamber; it did not guarantee that a majority set would ever be there. In fact, the mechanism for choosing legislators led to the downfall of the Parliamentary system in Paxos. Because of a scribe’s error, a decree that was supposed to honor sailors who had drowned in a shipwreck instead declared them to be the only members of Parliament. Its passage prevented any new decrees from being passed— including the decrees proposed to correct the mistake. Government in Paxos came to a halt. A general named Λαµπσων took advantage of the confusion to stage a coup, establishing a military dictatorship that ended centuries of progressive government. Paxos grew weak under a series of corrupt dictators, and was unable to repel an invasion from the east that led to the destruction of its civilization.LAMPORT, P. 22 — §3.3.6