Solution
travel(X,Y,go(X,Y,Transport)) :- onestep(X,Y,Transport). travel(X,Y,go(X,Z,Transport,Path)) :- onestep(X,Z,Transport), travel(Z,Y,Path). onestep(X,Y,byCar) :- byCar(X,Y). onestep(X,Y,byTrain) :- byTrain(X,Y). onestep(X,Y,byPlane) :- byPlane(X,Y).