Challenge 3: Removing the { } Braces Around the Arithmetic Goal — Possible Solution
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broken_expr_demo.pl:

expr_rest(Acc, Sum) -->
    [plus], term(T),
    Acc1 is Acc + T,        % missing { } -- this is the bug
    expr_rest(Acc1, Sum).

Query:

    ?- phrase(expr(Sum), [1, plus, 2, plus, 3]).
    ERROR: is/2: Arithmetic: evaluation error / type error
    (or, depending on the system, simply: false.)

-- Explanation --
--
-- Without the surrounding { }, the DCG translator has no way to know
-- that Acc1 is Acc + T is meant to be run as an ordinary Prolog goal
-- rather than matched as a grammar terminal against the input list.
-- Every unbracketed goal in a --> body is treated as something that
-- should consume elements from the remaining input, the same as
-- [plus] or term(T) does -- so Prolog effectively tries to unify the
-- structure is(Acc1, +(Acc, T)) against whatever is left of the
-- input list at that point, which is nonsense: the input list
-- contains plain numbers and the atom plus, not is/2 terms. This
-- either fails outright (no match) or raises a type/arithmetic error,
-- depending on exactly what's left in the input when this point is
-- reached -- but never actually performs the intended addition,
-- because is/2 was never executed as Prolog code in the first place.

WHY THIS WORKS AS AN ANSWER
------------------------------
This removes exactly the { } braces the chapter's warn-box calls out,
then traces precisely why the DCG translator treats the bare is/2
goal as a grammar terminal to match against the input rather than
code to execute, directly explaining the resulting failure or error
rather than just asserting one occurs.