Challenge 3: Why is Requires a Bound Right Side and = Doesn't — Possible Solution
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SWI-Prolog session:

    ?- X is Y + 1.
    ERROR: Arguments are not sufficiently instantiated

-- Explanation, as a comment:
--
-- is has a genuinely different job than =. is must ACTUALLY COMPUTE
-- a real arithmetic result -- to evaluate `Y + 1` as real arithmetic,
-- the system needs a concrete NUMBER for Y right now, in order to add
-- 1 to it. If Y is still an unbound variable, there is no number to
-- add 1 to at all -- arithmetic evaluation has no way to "solve for"
-- an unknown quantity inside an expression; it can only compute with
-- values it already has. This is exactly why is throws an error here
-- rather than somehow succeeding.
--
-- = has no such requirement, because unification's whole job is
-- different: it's asking "CAN these two terms be made identical,"
-- which is a question that can genuinely be answered even when one
-- or both sides are still unbound variables -- unification's own
-- answer, in that case, is simply "yes, by binding/linking them,"
-- per prolog1-3's own central material. Unification doesn't need a
-- concrete number to succeed; arithmetic evaluation genuinely does,
-- since "compute a result" and "check if two things could be made
-- equal" are fundamentally different operations with different
-- requirements.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the real "not sufficiently instantiated" error is
produces against an unbound variable, and the explanation correctly
traces the restriction back to the fundamental difference between
computing a concrete arithmetic result (which requires real numbers)
and unification (which can succeed on unbound variables by linking
them), tying back to prolog1-3's own central unification material.