* Because DIP is stored as a DDG object, this class is to record the test ids of all tests in the suite, which produce the same DIP.
** Although a DDG node offers both adjacency set and reverse adjacency set, DDG developers can choose to activate (allocate memory) either of them to use. When a set is not activated, it remains a NULL pointer, hence no memory is wasted for it. A union structure applied to DNode class tags either value A (in case of adjacency set) or value R (in case of reverse adjacency set) to each DNode object.