Dijkstra's paper, which introduces the concept of self-stabilization, presents an example in the context of a "token ring"—a network of computers ordered in a circle. Here, each computer or processor can "see" the whole state of one processor that immediately precedes it and that this state may imply that the processor "has a token" or it "does not have a token." One of the requirements is that exactly one of them must "hold a token" at any given time. The second requirement prescribes that each node "passes the token" to the computer/processor succeeding it so that the token eventually circulates the ring.