dfs(X, [X]) :- goal(X).
dfs(X, [X | L]) :- successor(X,S),dfs(S, L).

goal(g).
successor(s, a).
successor(s, b).
successor(a, b).
successor(a, c).
successor(b, c).
successor(b, d).
successor(c, d).
successor(c, g).