Try this online Prolog (Was: --- OLCOTT BANNED ---)

Hi,
 > - Its not performed in the goal to head unification
 > - Its not performed in (=)/2 buit-in
 > - It is performed in unify_with_occurs_check/2 built-in
An example of goal to head unification would be:
equal(X, X) :- write('You hit me!'), nl.
?- equal(foo(Y), Y).
You can try the same here with this online Prolog:
Example 02: Website Sandbox
https://www.xlog.ch/runtab/doclet/docs/04_tutor/reference/example02/package.html
It will give you:
You hit me!
Y = <cyclic term> .
Bye
Mild Shock schrieb:

You wrote:
  > Yet C&M say that this test is only performed when run
 > thus almost never performed because it is too expensive.
 But for soundness you have to perform it sometimes!
So what is the solution? The solution is simple:
 - Its not performed in the goal to head unification
- Its not performed in (=)/2 buit-in
- It is performed in unify_with_occurs_check/2 built-in
 The ISO core standard requires both (=)/2 and
unify_with_occurs_check/2. You need unify_with_occurs_check/2
when you want an occurs check, which is need
 for example in rewriting or type inference
to get correct results.