Hi Jim,

When I try to use the package in the subject heading I get

{ERROR: absolute_file_name/7, arg 1 - existence error: source_sink:library (use_occurs_check) does not exist}

what should I be using in order to have access to the predicate

unify_with_occurs_check/2 ?

This depends on your context. In the top level (which loads by default 'iso-misc') you do not need to do anything: it is loaded by default.

?- unify_with_occurs_check(X,Y).

Y = X ?

yes ?- unify_with_occurs_check(X,f(X)).

no ?-

The same happens in a module that uses a traditional module declaration, e.g.:

:- module(_,_).

p(X,Y) :- unify_with_occurs_check(X,Y).

since those modules load the same packages as the top level (the set of 'builtins' that is normally provided by standard Prologs). The same happens also in a "user" file (a file with no module declaration): those also load the same modules as the top level.

If you specify packages in the module declaration (including 'no packages', as below), then you have to load iso-misc explicitly:

:- module(_,_,[]).

:- use_module(library(iso_misc)).

p(X,Y) :- unify_with_occurs_check(X,Y).

In this mode one can control specifically which modules and packages are loaded and which not so that, e.g., one can make sure that no unwanted predicates (e.g., assert/retract, higher-order, side-effects, ...) are visible from a given module.

How to figure out that unify_with_occurs_check/2 is in iso_misc?

- In the manual: look in the index for unify_with_occurs_check/2 and it points you to iso_misc.

- Much easier: in the graphical environment. E.g., within emacs, in a .pl file (or the top level) put the cursor on unify_with_occurs_check and do 'C-c TAB', and this will take you to the manual page. The module is on the emacs mode line.

Hope this helps.

Manuel H