path(booshehr,shiraz).
path(shiraz,esfahan).
path(esfahan,tehran).
path(tehran,mashhad).
path(shiraz,yazd).
path(yazd,mashhad).
path(shiraz,tabas).

travel(X,Y) :- path(X,Y).
travel(X,Y) :- path(X,Z),
	       print(Z),nl,
	       travel(Z,Y).

% -----------------------------------------

pathbi(X,Y) :- path(X,Y);
               path(Y,X).

travelbi_user(X,Y) :- retractall(visited(Z)),
	              asserta(visited(X)),
		      asserta(visited(Y)),
		      travelbi(X,Y).

travelbi(X,Y) :- pathbi(X,Y).
travelbi(X,Y) :- pathbi(X,Z),
	         not(visited(Z)),
		 asserta(visited(Z)),
	         print(Z),nl,
	         travelbi(Z,Y).