Minimal Ubuntu installation and working example of graph exploration with Prolog with DAG traversal and full console output

graph.pl

% sudo apt-get install software-properties-common
% sudo apt-add-repository ppa:swi-prolog/stable
% sudo apt-get update
% sudo apt-get install swi-prolog

edge(1,2)
edge(2,3)
edge(3,4)
edge(2,6)
edge(6,7)

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

allPaths(A,B,LIST) :- findall(X, path(A,B,X), LIST).