Challenge 2: = Links, == Doesn't — Possible Solution
====================================================================

SWI-Prolog session:

    ?- X = Y.
    X = Y.

    ?- X == Y.
    false.

(Run as two separate, independent queries -- each starts with fresh,
unbound X and Y.)

Explanation:
`X = Y` performs unification between two unbound variables, which
succeeds by LINKING them together -- from that point on, binding
either one would bind the other too. `X == Y`, run as a completely
separate query with its own fresh X and Y, checks whether the two
terms are ALREADY structurally identical, with no unification
happening at all. Two distinct, unbound variables are NOT identical
to each other just because they're both unbound -- they're still two
genuinely separate variables, so == correctly reports false. The key
distinction: = actively changes the state of the variables (linking
them); == only inspects their current state, changing nothing.

WHY THIS WORKS AS AN ANSWER
------------------------------
This runs = and == as two separate queries on fresh unbound
variables, producing the exact different results the chapter
describes, and the explanation correctly identifies that = has a real
side effect (linking) while == has none.