% THIS IS the definition of allof relation from the
% textbook.  Store it in a file (say, allof).
% To include 'allof' it in the main file F, use 
% :- include('allof').




:- dynamic previndex/1.
previndex(0).

newindex(I1) :-
   retract(previndex(I)),
   I1 is I+1,
   assert(previndex(I1)).

% accumulate(Goal,Init,Prev,Acc,Next,Result)
% accumulates information for each success of Goal.
% Init is the start. Acc is a predicate that shows
% how to derive Next accumulation from Prev
% accumulation. Result is the final accumulation.

accumulate(G,I,P,CN,N,Res) :-
   newindex(Ind),
   accumulate(Ind,G,I,P,CN,N,Res).

accumulate(Ind,G,I,P,CN,N,_) :-
   assert(result(Ind,I)),
   call(G),
   retract(result(Ind,P)),
   call(CN),
   assert(result(Ind,N)),
   fail.
accumulate(Ind,_,_,_,_,_,Res) :-
   retract(result(Ind,Res)).

% allof(X,G,Res) is true if Res is the list of all X such that goal G is true
allof(X,G,Res) :-
   accumulate(G,L-L,P1-[X|P],true,P1-P,Res-[]).