(Upon the suggestion of @repeat) Consider a query of a pure program1 ?- G_0. What use if any would the query ?- G_0, G_0. have?
Footnotes
1 No tabling (to be safe), constraints are OK.
Previous post on the subject.
The query ?- G_0, G_0. helps to identify redundant answers of ?- G_0.
To do so it suffices to compare the number of answers of ?- G_0. with the number of answers of ?- G_0, G_0.. No need to store those answers (which is a frequent source of errors anyway). Just two integers suffice! If they are equal, then there is no redundancy. But if ?- G_0, G_0. has more answers, then there is some redundancy. Here is an example:
p(f(_,a)).
p(f(b,_)).
?- p(X).
X = f(_A, a)
; X = f(b, _A). % two answers
?- p(X), p(X).
X = f(_A, a)
; X = f(b, a)
; X = f(b, a)
; X = f(b, _A). % four answers
% thus p(X) contains redundancies
... and now let's fix this:
p(f(B,a)) :-
dif(B, b).
p(f(b,_)).
?- p(X).
X = f(_A, a), dif(_A, b)
; X = f(b, _A).
?- p(X), p(X).
X = f(_A, a), dif(_A, b), dif(_A, b).
; X = f(b, _A). % again two answers, thus no redundancy
No need to manually inspect the constraints involved.
This can be further extended when we are explicitly searching for redundant answers only using call_nth/2.
?- G_0, call_nth(G_0, 2).