Proof-of-diversity rule 1: After the first existing verifier in the block chain, a new verifier is only allowed if none of the other blocks in the cycle, the previous cycle, or the two blocks before the previous cycle were verified by new verifiers.

This rule exists to ensure that attackers cannot gain control of the chain by quickly introducing verifiers that they control. The addition of the two blocks before the previous cycle is to avoid clustering new verifiers in the chain.