Subgraph = class

There are several the same subgraphs in a large graph can be overlapped to be only one in the memory as a class, and then the boundaries between the overlapped subgraph and the large graph holes formed after the subgraphs cutted can be regarded as the instances of this class.

There may also be several the same subgraphs in a subgraph. Travelers remember one more boundary each time crossing a boundary into a subgraph, and forget the last added boundary when exiting a subgraph. Via these remembered boundaries, the travelers from different instances can go back where they came after they leave the class traveling. 

It is possible to have different subgraph within an overlapped graph, in this condition, there are several different boundaries at the same place.