A system for modifying a rule base for use in processing data, wherein an instance of an entity comprises associated data, for use with a parser for identifying a first instance of an entity in data associated with a document set in accordance with a first rule and for using the first instance of the entity to identify a second instance of the entity in the data associated with the document set. The system comprises an accessor for accessing data associated with the second instance of the entity; and a generator for using the data associated with the second instance of the entity to generate a second rule.