 As David Lewis noted in his book Convention (1969) many prominent features of social life depend on agents assuming that the rules of some practice are matters of common knowledge. For example, drivers know that a red traffic light indicates that they should stop at an intersection. However, for the convention of traffic lights to be in place at all, it is first necessary that drivers must also know that other drivers know that red means stop. In addition, drivers must also know that everyone knows that everyone knows that …. The conventional role of traffic lights relies on all drivers knowing that all drivers know the rule, that the rule is a piece of  common knowledge.