Using attribute_goal/2 the right way in SICStus Prolog

Question

I'm writing a "solver" using the SICStus Prolog attributed variables interface:

:- module(attach2, [attach2/2]).
:- use_module(library(atts)).
:- attribute att/2.

attach2(X,Y) :- put_atts(X,att(X,Y)),
                put_atts(Y,att(Y,X)).

verify_attribute(_,_,[]).

attribute_goal(V,(true,G,true,G,true)) :- get_atts(V,att(X,Y)), G = attach2(X,Y).

Sample query:

| ?- attach2(X,Y), copy_term(X,Xc,Xcc), copy_term(Y,Yc,Ycc), copy_term(X+Y,XYc,XYcc).
Xcc = attach2:(true,attach2(Xc,_A),true,attach2(Xc,_A),true),
Ycc = attach2:(true,attach2(Yc,_B),true,attach2(Yc,_B),true),
XYc = _C+_D,
XYcc = (attach2:attach2(_C,_D),attach2:attach2(_D,_C)),
attach2(X,Y),
attach2(Y,X) ? ;
no

If I change attribute_goal/2 to ...

attach2(X,Y) :- put_atts(X,att(X,Y)),
                put_atts(Y,att(X,Y)).

... the answer get better:

| ?- attach2(X,Y), copy_term(X,Xc,Xcc), copy_term(Y,Yc,Ycc), copy_term(X+Y,XYc,XYcc).
Xcc = attach2:(true,attach2(Xc,_A),true,attach2(Xc,_A),true),
Ycc = attach2:(true,attach2(_B,Yc),true,attach2(_B,Yc),true),
XYc = _C+_D,
XYcc = attach2:attach2(_C,_D),
attach2(X,Y) ? ;
no

My conclusion from this:

copy_term/3 collects goals from attribute_goal/2 and if it encounters more than one attributed variable, then it removes trivial true and duplicate goals.

It this essentially right?