Unification with STO detection

Question

In ISO Prolog unification is defined only for those cases that are NSTO (not subject to occurs-check). The idea behind is to cover those cases of unifications that are mostly u

Answer 1

In SWI-prolog:

unify_sto(X,Y) :-
  \+ unify_with_occurs_check(X,Y),
  X = Y,
  !,
  writeln('Error: NSTO failure'),
  fail.

unify_sto(X,Y) :-
  X = Y.

gives the following results:

[debug]  ?- unify_sto(X,s(X)).
Error: NSTO failure
false.

[debug]  ?- unify_sto(X,a).
X = a.

[debug]  ?- unify_sto(b,a).
false.