. . "This set is a superset of any other set in the DAG. Additionally, each DAG is augmented with all non-empty intersections S1???S2, of sets S1 and S2 in the DAG. This ensures that the DAG obeys a ???closure??? property, whereby the non-empty intersection of any two sets in the DAG is itself a set in the DAG. It can thus be seen that each set in the DAG is either the root set, one of the transaction" . .