WikiMath

  trainDirect(forbach,saarbruecken).
  trainDirect(freyming,forbach).
  trainDirect(fahlquemont,stAvold).
  trainDirect(stAvold,forbach).
  trainDirect(saarbruecken,dudweiler).
  trainDirect(metz,fahlquemont).
  trainDirect(nancy,metz).

  allerDe(X,X).
  allerDe(X,Y) :- trainDirect(X,Z), allerDe(Z,Y).