Invariant that the previous certificates referenced by a certificate in a DAG are also in the DAG.
A validator's DAG is extended via two kinds of events,
namely
The first kind of event may only occur if the certificate's author has all the previous certificates in its DAG.
The second kind of event may only ocur if the DAG has all the previous certificates. Under that condition, the certificate is moved from the buffer to the DAG. In fact, the purpose of the buffer is to wait until all the previous certificates are in the DAG, since they may be coming out of order with respect to round numbers.