Here is a route planning program given by an online tutorial:-
route(X,Y,R) :-
route(X,Y,[X],R).
route(X,Y,_,[drive(X,Y)]) :-
travel(X,Y).
route(X,Y,V,[drive(X,Z)|R]) :-
travel(X,Z),
\+ member(Z,V),
route(Z,Y,[Z|V],R)
Z \= Y. %Only required if Y is not ground.
travel(X,Y) :- road(X,Y).
travel(X,Y) :- road(Y,X).
road(arad,sibiu).
road(arad,timisoara).
road(arad,zerind).
road(zerind,oradea).
road(oradea,sibiu).
road(sibiu,fagaras).
What I do not understand is the commented statement: Z\=Y.
Why this statement is needed? And why that statement is only needed if Y is not ground?
