% -*-Prolog-*- % thisis similar to the sum example. % given a set of assertions of the form: % % income(person,source,year,$) % % this rule will maintain a set of yearly totals of the form % % total_income(person,year,$) % income(Person,Source,Year,Dollars) => {increment_income(Person,Year,Dollars)}. => fcUndoMethod(increment_income(P,Y,D),decrement_income(P,Y,D)). increment_income(P,Y,D) :- (retract(total_income(P,Y,Old)) -> New is Old+D ; New = D), assert(total_income(P,Y,New)). decrement_income(P,Y,D) :- retract(total_income(P,Y,Old)), New is Old-D, assert(total_income(P,Y,New)).