Differences among the "bomb" and "xbetween" (Was: Request for comments, Novacore the sequel to ISO modules)

Here are two test cases for memory
management of a Prolog system:
/* bomb */
app([], X, X).
app([X|Y], Z, [X|T]) :- app(Y, Z, T).
garbage(0, [0]) :- !.
garbage(N, L) :- M is N-1, garbage(M, R), app(R, R, L).
foo :- garbage(12,_), foo.
/* xbetween */
xbetween1(L, _, L).
xbetween1(L, U, N) :- L < U, M is L+1, xbetween1(M, U, N).
They test possibly something different. xbetween does
not produce a lot of objects during tail recursion,
it only decrements one integer. The xbetween example
might be ok, wherea the bomb example might be neverthelesss
not ok, especially since unlike in the xbetween example,
the bomb example has also an "intermediate" variables.
The "intermediate" variable is "_":
foo :- garbage(12,_), foo.
The xbetween example has no such variable. All
variables in the xbetween example are either in
the head or in the tail recursive call, making
it a more trivial example than the bomb example.