A method for applying a rule to data includes receiving data from a data source and processing a particular data unit of an ordered number of data units derived at least in part from the received data. The processing includes determining a set of data units including an ordered subset of data units from the ordered number of data units, each data unit of the ordered subset being related to the particular data unit and occurring before the particular data unit in the ordered number of data units. Prior to applying the rule the state variable is updated to a state that would result from iterating in order through the set of data units and, for each data unit of the ordered subset, applying the rule to the data unit. The rule is then applied to the particular data unit including reading the updated value of the state variable.