General
Material
Lecture 1
Lecture 2
Lecture 3
Lecture 4
Lecture 5

Solution

travel(X,Y) :- onestep(X,Y).
travel(X,Y) :- onestep(X,Z),
               travel(Z,Y).

onestep(X,Y) :- byCar(X,Y).
onestep(X,Y) :- byTrain(X,Y).
onestep(X,Y) :- byPlane(X,Y).
    

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