Another Recursive Query

• Flight(x,y) : there is a flight from city x to y.

• Hub(x): x is a hub.

• Query: find pairs of cities for which there is a flight that goes through some hub.

Fl(x,y) :- Flight(x,y)

Fl(x,y) :- Flight(x,z), Fl(z,y)

Answer(x,y) :- Fl(x,z), Hub(z), Fl(z,y).

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