%% Given facts.
directTrain(saarbruecken,dudweiler).
directTrain(forbach,saarbruecken).
directTrain(freyming,forbach).
directTrain(stAvold,freyming).
directTrain(fahlquemont,stAvold).
directTrain(metz,fahlquemont).
directTrain(nancy,metz).

%% Rules.
travelFromTo(X, Y) :- directTrain(X, Y).
travelFromTo(X, Y) :- directTrain(X, Z),
                      travelFromTo(Z, Y).