Challenge 2: A Missing Route Isn't Proof of Impossibility — Possible Solution
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flights.pl:

flight(paris, london).
flight(london, paris).
flight(paris, rome).

Query:

    ?- \+ flight(london, tokyo).
    true.

-- Why doesn't this prove such a flight is impossible? --
--
-- This query succeeds purely because flight(london, tokyo) cannot be
-- PROVEN from the three facts actually loaded into this database --
-- none of them mention tokyo at all. Under the closed-world
-- assumption, Prolog treats "cannot be proven" and "false" as the
-- exact same thing, with no separate category for "simply not
-- recorded here." In reality, this flights.pl file is obviously a
-- tiny, incomplete sample of real-world flight data -- there could
-- easily be a genuine London-Tokyo route that just never got entered
-- as a fact. The query result only reflects what THIS database
-- happens to contain, not any fact about whether such a flight
-- actually exists in the world. Absence of evidence (no matching
-- fact) is being reported as if it were evidence of absence (no such
-- flight exists), which is exactly the real, well-documented
-- limitation this chapter names.

WHY THIS WORKS AS AN ANSWER
------------------------------
This runs a \+ query that succeeds only because of the database's own
incompleteness, then explicitly names the closed-world assumption as
the reason a "true" result here cannot be read as a genuine real-world
fact, directly demonstrating the chapter's own absence-of-evidence
warning with a fresh example rather than reusing the chapter's own
tokyo query verbatim.