people.pl

ｐｒｏｌｏｇ样例

right_to(martin,lisa).   right_to(lisa,george).
right_to(george,clara).  right_to(clara,adam).
right_to(adam,susan).    right_to(susan,martin).

left_to(X,Y) :- right_to(Y,X).

neighbour(X,Y,Z) :- right_to(Y,Z),left_to(Y,X).

guests:-nl,right_to(X,_), write(X), nl, fail.
guests.