Express the query of Exercise 13.19 in Datalog.

What will be an ideal response?

First, we de?ne the transitive closure of DirectFlight:

Flight(?F,?T) :- DirectFlight(?F,?T).

Flight(?F,?T) :- DirectFlight(?F,?I), Flight(?I,?T).

Then we de?ne the required query:

FlightsWithTwoOrMoreStops(?F,?T) :-

                                    DirectFlight(?F,?S1),

                                    DirectFlight(?S1,?S2),

                                    Flight(?S2,?T).