Challenge 2: A Recursive ancestor/2 Spanning Three Generations — Possible Solution
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family.pl:

parent(george, susan).
parent(susan, tom).
parent(tom, alice).

ancestor(X, Y) :- parent(X, Y).
ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).

Query:

    ?- ancestor(george, alice).
    true.

Explanation:
george is not alice's direct parent, so the base case
(ancestor(X, Y) :- parent(X, Y)) does not match directly for
ancestor(george, alice). The recursive case fires instead:
parent(george, Z) binds Z = susan, and the rule then needs
ancestor(susan, alice) to also hold. THAT itself isn't a direct
parent relationship either, so the recursive case fires AGAIN:
parent(susan, Z2) binds Z2 = tom, requiring ancestor(tom, alice) --
which now genuinely IS a direct parent fact, satisfying the base
case at last. The recursion "unwinds" back up through each level,
confirming ancestor(george, alice) overall, exactly the kind of
multi-generation reach the chapter's own base-case/recursive-case
shape is designed to handle.

WHY THIS WORKS AS AN ANSWER
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This uses the chapter's own recursive ancestor/2 definition over a
genuine three-generation gap (requiring the recursive case to fire
twice before the base case resolves it), confirming the recursion
correctly reaches beyond a single generation.