Challenge 3: What Breaks When a Hole Is Closed Too Early — Possible Solution
====================================================================

dl_demo.pl:

dl_concat(A-B, B-C, A-C).

Query:

    ?- L1 = [a, b | H1], DL1 = L1-H1,
       H1 = [],                      % hole closed early, by mistake
       L2 = [c, d | H2], DL2 = L2-H2,
       dl_concat(DL1, DL2, Combined).
    false.

-- Explanation --
--
-- After H1 = [], DL1 is no longer an open difference list at all --
-- it has become the ordinary, fully closed list [a, b]-[], since its
-- own hole was bound to [] before any splicing happened. When
-- dl_concat(DL1, DL2, Combined) then runs, it tries to unify DL1's
-- own second component (which is now permanently [], not an
-- unbound variable) with DL2's first component (the list [c, d | H2]).
-- Unifying [] with [c, d | H2] fails outright -- [] and a
-- non-empty list can never unify -- so the whole dl_concat call
-- fails, with no useful error message pointing at H1 = [] as the
-- actual root cause several lines earlier. The list [a, b] is now
-- permanently sealed off; nothing can ever be spliced into it again.

WHY THIS WORKS AS AN ANSWER
------------------------------
This deliberately closes a difference list's hole before attempting a
second concatenation, showing the resulting dl_concat/3 call fails
outright because [] can no longer unify with an incoming list,
directly demonstrating the chapter's own warning about premature
hole-binding silently breaking later concatenations.