Challenge 2: Counting All 5-Queens Solutions via findall/3 — Possible Solution
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queens_demo.pl (same queens/2, safe_queens/1, safe_queens/3 as Challenge 1)

Query:

    ?- findall(Qs, queens(5, Qs), AllSolutions), length(AllSolutions, Count).
    Count = 10.

Explanation:
findall(Qs, queens(5, Qs), AllSolutions) drives queens(5, Qs) to full
exhaustion via backtracking -- every possible way label/1 can commit
to concrete values while still satisfying every #\= and diagonal
constraint safe_queens/1 posted -- collecting each resulting Qs list
into AllSolutions in one call, exactly the way prolog2-1 described
findall gathering every solution of a goal into a single concrete
list. length/2 then simply counts how many were found. 10 is the
well-known correct number of distinct solutions to the 5-queens
problem (matching the classical, independently-verified result for
N=5), confirming both that queens/2 is complete -- it doesn't miss
any valid placement -- and that findall correctly exhausts every one
of them rather than stopping early.

WHY THIS WORKS AS AN ANSWER
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This reuses the chapter's own findall-based counting pattern (already
shown for N=6) at a different N, and cross-checks the resulting count
against the independently well-known correct value for 5-queens,
confirming both the solver's completeness and findall's own
exhaustive-collection behavior.