 Here, arrows represent a binary relation on possible worlds. In modal logic in general, it is referred to as the accessibility relation. Under the indistinguishability interpretation of epistemic logic, it is sometimes called the indistinguishability relation. Formally, denote the relation \(R_{a}\), with the subscript showing the relation belongs to agent a. The relation is a subset of the set of ordered pairs of possible worlds, \(\{(w,w')\colon w,w'\in W\}\). One world w “points” to another \(w'\) if \((w,w')\in R_{a}\). In this case, \(w'\) is said to be accessible (indistinguishable) from w. In the literature, this is often written \(wR_{a}w'\) or \(R_{a}ww'\). The notation ‘\(w'\in R_{a}(w)\)’ is also common: the set \(R_{a}(w)\) is then the worlds accessible from w, i.e., 