/* prolog tutorial 2.15 Graph structures and paths */

edge(1,2).
edge(1,4).
edge(1,3).
edge(2,3).
edge(2,5).
edge(3,4).
edge(3,5).
edge(4,5).

connected(X,Y) :- edge(X,Y) ; edge(Y,X).

path(A,B,Path) :-
       travel(A,B,[A],Q), 
       reverse(Q,Path).

travel(A,B,P,[B|P]) :- 
       connected(A,B).
travel(A,B,Visited,Path) :-
       connected(A,C),           
       C \== B,
       \+member(C,Visited),
       travel(C,B,[C|Visited],Path).  

% ?- path(1,5,P).