 Bisimulation between graphs. Before giving examples, we should clarify some usage. At a few points, we’ll speak of bisimulation between two graphs G and H, rather than on a single graph.    This can be defined in the same general way.    Note also that one can    take the disjoint union G + H     of the graphs G and H, and then     a bisimulation between G and H would be a  bisimulation on G + H. 