 One interpretation of events regards them as the production of values in the evaluation of an expression. This interpretation originates in the context of bottom-up computation of recursive programs developed by Berry (1976), where a recursive definition is translated into a graph displaying the dependence of results of an expression on results of its subexpressions. This context naturally suggests the notion of producer of an event \(x\), as a set of events that must have happened in order that \(x\) may happen. Reformulating this observation in the language of partial orders, Berry (1976) defined: