dates(john,nancy).
dates(john,susan).
takes(john,vacation) :- iss(john,doomed).
knows(nancy, (dates(john,nancy) , dates(john,susan))) :- dates(john,nancy),dates(john,susan).
iss(john,doomed):- 	knows(nancy, (dates(john,nancy) , dates(john,susan))) ,
  					knows(susan, (dates(john,nancy) , dates(john,susan))).
leaves(john,country) :- takes(john,vacation).

/* TYPE leaves(john,country). */
/* The answer is false because Susan does not know */
/* if we add knows(susan, (dates(john,nancy) , dates(john,susan))).  the asnwer will be true*/