My stupid Dogelog Player falls back to _<number> (Was: variable ordering guarantees in term_singletons/2)

SWI-Prolog has quite an elaborate '_' determiner:
/* SWI-Prolog */
?- length(L,2), (R = L; length(R,2)).
L = R, R = [_, _] ;
L = [_, _],
R = [_, _].
?- length(L,2), (R = f(L); length(R,2)).
L = [_A, _B],
R = f([_A, _B]) ;
L = [_, _],
R = [_, _].
Especially the L = R detection, seems challenging.
Thats something more complex than my listing/[0,1]
and term_singletons/2 usage there.
I have no pure Prolog solutions yet. My stupid
Dogelog Player falls back to _:
/* Dogelog Player */
?- length(L,2), (R = L; length(R,2)).
L = [_309342, _309344], R = [_309342, _309344];
L = [_309342, _309344], R = [_309495, _309497].
?- length(L,2), (R = f(L); length(R,2)).
L = [_310399, _310401], R = f([_310399, _310401]);
L = [_310399, _310401], R = [_310574, _310576].
Mild Shock schrieb:

Hi,
 People from Vienna were always a little strange.
Why not adopt term_singletons/2 its already around
for a while. You can do quite some magic with it.
 Example: Determining singletons during listing,
from Dogelog Player library(tester/tools):
 % sys_listing_write(+Term, +Stream)
sys_listing_write(C, T) :-
    term_variables(C, V),
    term_singletons(C, A),
    sys_listing_names(V, A, 0, N),
    write_term(T, C, [quoted(true), variable_names(N), format(true)]),
    sys_answer_period(T).
 If term_singletons/2 has the same variable ordering
guarantees, i.e. left to right, as in term_variables/2
you can use an algorithm without expensive lookup,
 sys_listing_names([], _, _, []).
sys_listing_names([X|L], [Y|R], K, ['_'=X|S]) :- X==Y, !,
    sys_listing_names(L, R, K, S).
sys_listing_names([X|L], A, K, [N=X|R]) :-
    sys_listing_name(K, N),
    J is K+1,
    sys_listing_names(L, A, J, R).
 Just run along the two lists , if something is both in
the term_variables/2 and term_singletons/2 list, generate
a '_' name, otherwise generate a synthetic name.
 Bye
 Mild Shock schrieb:

Hi,
>
The development of Trealla Prolog and Scryer Prolog
looks like a random search in a mental ocean.
>
It is like Prolog Development à la Sigmund Freud,
you have only to dig deep enough, and a solution
>
will pop up. Otherwise blame your mother or other
relatives that raised you for supression.
>
LoL
>
Bye
>
Examples: Still clueless how to detect singletons?
https://github.com/trealla-prolog/trealla/issues/743