Challenge 1: All Color/Size Combinations via Backtracking — Possible Solution
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facts.pl:

color(red).
color(green).
color(blue).

size(small).
size(medium).
size(large).

Query:

    ?- color(X), size(Y).
    X = red,
    Y = small ;
    X = red,
    Y = medium ;
    X = red,
    Y = large ;
    X = green,
    Y = small ;
    X = green,
    Y = medium ;
    X = green,
    Y = large ;
    X = blue,
    Y = small ;
    X = blue,
    Y = medium ;
    X = blue,
    Y = large.

Explanation:
color(X) has three possible answers (red, green, blue) and size(Y)
has three possible answers (small, medium, large) -- combined in a
single conjunction, Prolog generates all nine combinations by
backtracking: it fixes X = red, exhausts every Y value, THEN
backtracks into color(X) for the next value (green), exhausting every
Y again, and so on. Pressing ; after each answer requests exactly
this next combination, with no explicit nested loop written anywhere
in the code.

WHY THIS WORKS AS AN ANSWER
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This reproduces the chapter's own member/2 combination example using
two independent sets of facts, confirming backtracking generates
every combination of the two automatically.