%%% The London Underground example %%%

connected(bond_street,oxford_circus,central).
connected(oxford_circus,tottenham_court_road,central).
connected(bond_street,green_park,jubilee).
connected(green_park,charing_cross,jubilee).
connected(green_park,piccadilly_circus,piccadilly).
connected(piccadilly_circus,leicester_square,piccadilly).
connected(green_park,oxford_circus,victoria).
connected(oxford_circus,piccadilly_circus,bakerloo).
connected(piccadilly_circus,charing_cross,bakerloo).
connected(tottenham_court_road,leicester_square,northern).
connected(leicester_square,charing_cross,northern).


nearby(X,Y):-connected(X,Y,L).
nearby(X,Y):-connected(X,Z,L),connected(Z,Y,L).


reachable(X,Y,[]):-connected(X,Y,L).
reachable(X,Y,[Z|R]):-connected(X,Z,L),reachable(Z,Y,R).


line(central,[bond_street,oxford_circus,tottenham_court_road]).
line(jubilee,[bond_street,green_park,charing_cross]).
line(piccadilly,[green_park,piccadilly_circus,leicester_square]).
line(victoria,[green_park,oxford_circus]).
line(bakerloo,[oxford_circus,piccadilly_circus,charing_cross]).
line(northern,[tottenham_court_road,leicester_square,charing_cross]).