% From The Art of Eyepl, Chapter 23.
traveler_in_good_standing(Person) :-
  registered(Person),
  identity_checked(Person),
  not(suspended(Person)).

can_board(Person) :-
  traveler_in_good_standing(Person),
  has_ticket(Person).

can_enter_lounge(Person) :-
  traveler_in_good_standing(Person),
  lounge_pass(Person).