docs: protected Edge buildConcreteMNEdge(Vertex vertex1, Vertex vertex2)
      {
          ModificationState state1 = vertex1.getEnvelope().getModificationState();
          ModificationState state2 = vertex2.getEnvelope().getModificationState();
          if (state1.needsUpdate() || state1.needsInsert())
          {
              if (state2.needsInsert())
              {
                  // (2) must be inserted before we can create a link to it
                  return new Edge(vertex2, vertex1, CONCRETE_EDGE_WEIGHT);
              }
          }
          else if (state1.needsDelete())
          {
              if (state2.needsDelete())
              {
                  // there is a link from (1) to (2) which must be deleted first,
                  // which will happen when deleting (1) - thus:
                  return new Edge(vertex1, vertex2, POTENTIAL_EDGE_WEIGHT);
              }
          }
          return null;
      }