road(berlin, potsdam). road(potsdam, werder). road(werder, brandenburg). road(X, Y) :- road(Y, X). blocked(werder, brandenburg). route(X, Y) :- road(X, Y), not blocked(X, Y). route(X, Y) :- route(X, Z), route(Z, Y). drive(X) :- route(berlin, X). #show drive/1.