% -*-Prolog-*-

:- expects_dialect(pfc).

% 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)}.

==> do_and_undo(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)).