/* Logical rules for passing an exam */

pass_exam(X) :- prepared_for_exam(X).

pass_exam(X) :- easy_exam, person(X).


/* Three ways to be prepared for exam */

prepared_for_exam(X) :-
    knows_it_all(X).

prepared_for_exam(X) :-
    read_book(X),
    attended_lectures(X),
    \+ slept_during_lectures(X),
    worked_problems(X).

prepared_for_exam(X) :-
    tutored_by(X, Y),
    prepared_for_exam(Y).




:- dynamic easy_exam/0, attended_lectures/1, slept_during_lectures/1.


/* The facts */

person(mary).
person(john).
person(tom).
person(sally).
person(fred).

read_book(fred).
read_book(mary).

worked_problems(fred).
worked_problems(mary).

attended_lectures(fred).
attended_lectures(mary).

slept_during_lectures(fred).

knows_it_all(tom).

tutored_by(john, mary).
tutored_by(sally, john).


% auxiliary routines

% show shows an enumerated relation, one tuple per line

show(Relation) :-
  enumerate(Relation, List),
  list(List).


% enumerate enumerates a named relation to produce a list

enumerate(Relation, List) :-
  current_predicate(Relation, Form),
  Form =.. [Relation | Args],
  (setof(Args, Form, List) ; List = []).


% list shows a list, one element per line

list([]).
list([A | X]) :-
  write(A),
  nl,
  list(X).