 For instance , a Bayesian network represents a system of <unk> events as vertices in a directed acyclic graph , in which the likelihood of an event may be calculated from the <unk> of its predecessors in the DAG . In this context , the moral graph of a DAG is the undirected graph created by adding an ( undirected ) edge between all parents of the same vertex ( sometimes called marrying ) , and then replacing all directed edges by undirected edges . 
