Challenge 2: Finding All Red Fruits With ; — Possible Solution
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SWI-Prolog session:

    ?- assertz(color(apple, red)).
    true.

    ?- assertz(color(cherry, red)).
    true.

    ?- color(X, red).
    X = apple ;
    X = cherry.

Explanation:
Two separate facts both state that a fruit is red -- apple and
cherry. The single query color(X, red) asks "for what X is
color(X, red) true?", and because TWO facts genuinely satisfy that
question, Prolog reports the first (X = apple) and then waits.
Pressing ; asks explicitly for another solution, and Prolog searches
further through its database, finding cherry as well. This is the
exact multiple-answers behavior the chapter previews as a first taste
of backtracking.

WHY THIS WORKS AS AN ANSWER
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This reproduces the chapter's own multiple-facts-multiple-answers
example directly, with genuinely two facts satisfying the same query
and both answers retrieved in turn via ;.