Scusatemi, ho trovato questo sistema esperto sulla rete:
% Native - a simple shell for use with Prolog
% knowledge bases. It includes expanations.
:-op(900,xfy, :).
main :-
greeting,
repeat,
write('> '),
read(X),
do(X),
X == quit.
greeting :-
write('This is the native Prolog shell.'), nl,
native_help.
do(help) :- native_help, !.
do(load) :- load_kb, !.
do(solve) :- solve, !.
do(how(Goal)) :- how(Goal), !.
do(whynot(Goal)) :- whynot(Goal), !.
do(quit).
do(X) :-
write(X),
write(' is not a legal command.'), nl,
fail.
native_help :-
write('Type help. load. solve. how(Goal). whynot(Goal). or quit.'),nl,
write('at the prompt.'), nl.
load_kb :-
write('Enter file name in single quotes (ex. ''birds.nkb''.): '),
read(F),
reconsult(F).
solve :-
abolish(known,3),
prove(top_goal(X),[]),
write('The answer is '),write(X),nl.
solve :-
write('No answer found.'),nl.
ask(Attribute,Value,_) :-
known(yes,Attribute,Value), % succeed if we know its true
!. % and dont look any further
ask(Attribute,Value,_) :-
known(_,Attribute,Value), % fail if we know its false
!, fail.
ask(Attribute,_,_) :-
not multivalued(Attribute),
known(yes,Attribute,_), % fail if its some other value.
!, fail. % the cut in clause #1 ensures
% this is the wrong value
ask(A,V,Hist) :-
write(A :V), % if we get here, we need to ask.
write('? (yes or no) '),
get_user(Y,Hist), % get the answer
asserta(known(Y,A,V)), % remember it so we dont ask again.
Y = yes. % succeed or fail based on answer.
% "menuask" is like ask, only it gives the user a menu to to choose
% from rather than a yes on no answer. In this case there is no
% need to check for a negative since "menuask" ensures there will
% be some positive answer.
menuask(Attribute,Value,_,_) :-
known(yes,Attribute,Value), % succeed if we know
!.
menuask(Attribute,_,_,_) :-
known(yes,Attribute,_), % fail if its some other value
!, fail.
menuask(Attribute,AskValue,Menu,Hist) :-
nl,write('What is the value for '),write(Attribute),write('?'),nl,
display_menu(Menu),
write('Enter the number of choice> '),
get_user(Num,Hist),nl,
pick_menu(Num,AnswerValue,Menu),
asserta(known(yes,Attribute,AnswerValue)),
AskValue = AnswerValue. % succeed or fail based on answer
display_menu(Menu) :-
disp_menu(1,Menu), !. % make sure we fail on backtracking
disp_menu(_,[]).
disp_menu(N,[Item | Rest]) :- % recursively write the head of
write(N),write(' : '),write(Item),nl, % the list and disp_menu the tail
NN is N + 1,
disp_menu(NN,Rest).
pick_menu(N,Val,Menu) :-
integer(N), % make sure they gave a number
pic_menu(1,N,Val,Menu), !. % start at one
pick_menu(Val,Val,_). % if they didn't enter a number, use
% what they entered as the value
pic_menu(_,_,none_of_the_above,[]). % if we've exhausted the list
pic_menu(N,N, Item, [Item|_]). % the counter matches the number
pic_menu(Ctr,N, Val, [_|Rest]) :-
NextCtr is Ctr + 1, % try the next one
pic_menu(NextCtr, N, Val, Rest).
get_user(X,Hist) :-
repeat,
write('> '),
read(X),
process_ans(X,Hist), !.
process_ans(why,Hist) :-
write_list(4,Hist), !, fail.
process_ans(X,_).
% Prolog in Prolog for explanations.
% It is a bit confusing because of the ambiguous use of the comma, both
% to separate arguments and as an infix operator between the goals of
% a clause.
prove(true,_) :- !.
prove((Goal,Rest),Hist) :-
prov(Goal,[Goal|Hist]),
prove(Rest,Hist).
prove(Goal,Hist) :-
prov(Goal,[Goal|Hist]).
prov(true,_) :- !.
prov(menuask(X,Y,Z),Hist) :- menuask(X,Y,Z,Hist), !.
prov(ask(X,Y),Hist) :- ask(X,Y,Hist), !.
prov(Goal,Hist) :-
clause(Goal,Body),
prove(Body,Hist).
% Explanations
how(Goal) :-
clause(Goal,Body),
prove(Body,[]),
write_body(4,Body).
whynot(Goal) :-
clause(Goal,Body),
write_line([Goal,'fails because: ']),
explain(Body).
whynot(_).
explain(true).
explain((Head,Body)) :-
check(Head),
explain(Body).
check(H) :- prove(H,[]), write_line([H,succeeds]), !.
check(H) :- write_line([H,fails]), fail.
write_list(N,[]).
write_list(N,[H|T]) :-
tab(N),write(H),nl,
write_list(N,T).
write_body(N,(First,Rest)) :-
tab(N),write(First),nl,
write_body(N,Rest).
write_body(N,Last) :-
tab(N),write(Last),nl.
write_line(L) :-
flatten(L,LF),
write_lin(LF).
write_lin([]) :- nl.
write_lin([H|T]) :-
write(H), tab(1),
write_lin(T).
flatten([],[]) :- !.
flatten([[]|T],T2) :-
flatten(T,T2), !.
flatten([[X|Y]|T], L) :-
flatten([X|[Y|T]],L), !.
flatten([H|T],[H|T2]) :-
flatten(T,T2).
Non ho capito bene cosa bisogna passare ad "ask" e cosa fa "how(Goal)". Il primo è strettamente correlato al secondo? Ed il secondo mi serve per capire come ho fatto ad arrivare al goal? 
