% demonstration of how backtracking in Prolog acts like a co-routine.
square(X,Y):- Y is X*X.
% The only way to store intermediate results in Prolog is to file them in the data base.
summer_setup:-asserta(sum_is_now(0)), !.
% This is the worker predicate - it pulls the total out of the data base(retract)
% and then adds it to the current X.  Ut then puts the NewSum in the data base.
summer(X):-retract(sum_is_now(S),_), !, 
		NewS is S + X,
		asserta(sum_is_now(NewS)).
% gen(X) generates three values, one after another...
gen(1). gen(2). gen(3).
trace [gen,summer, square,sum_is_now]!
% How to sum the gen-erated data: set up, then gen and accumulate.
% Run as a query - and each 'y' input adds in the next one.
dosum:- summer_setup, gen(I), summer(I), sum_is_now(S).
dosumsqr:- summer_setup, gen(I), J is I*I, summer(J), sum_is_now(S).
% Here is an alternate generator
getem(I):-repeat, prin("Input a number and a period > "),read(T),I is T.
% sum input data...
suminput:- summer_setup, getem(I), summer(I), sum_is_now(S).