Challenge 3: What Happens Without label/1 — Possible Solution
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Query, deliberately omitting label/1:

    ?- X in 1..9, Y in 1..9, X + Y #= 10.
    X in 1..9,
    Y in 1..9,
    X+Y#=10.

-- Explanation --
--
-- Without label/1, the query reports back the CURRENT STATE OF THE
-- CONSTRAINTS themselves -- the narrowed domains for X and Y, plus
-- the constraint connecting them -- not a concrete pair of numbers.
-- In this particular example the domains happen to stay 1..9 for
-- both variables (X + Y #= 10 alone doesn't narrow either domain
-- further without more constraints to combine it with), but even
-- when propagation genuinely does shrink a domain down significantly
-- (as in the chapter's own X #= Y + 1 example, where X's domain
-- narrowed to 2..6), the result is still a RANGE, never a single
-- committed value. This isn't yet a usable answer for anything
-- expecting an actual number -- X and Y remain domain variables,
-- constrained but not yet resolved. label([X, Y]) is the step that
-- actually searches through the remaining possibilities and commits
-- to one concrete pair at a time, which is why forgetting it is
-- flagged as a common, easy-to-miss mistake.

WHY THIS WORKS AS AN ANSWER
------------------------------
This runs the chapter's own example with label/1 deliberately
removed, shows the resulting domain-only output rather than concrete
numbers, and explains why that output -- even when propagation has
narrowed the domains -- still isn't a usable final answer without the
labeling step.