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Logtalk - Object oriented extension to Prolog
Release 2.9.3

Copyright (c) 1998-2002 Paulo Moura.  All Rights Reserved.
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To load all objects in this example consult the polygons.loader utility
file (note that the *.loader files are Prolog files).

You will need to load the objects in the roots and relations 
examples (consulting the corresponding roots.loader and relations.loader 
files).

You will also need to consult the following files in the library directory: 
events.loader, types.loader, metapredicates.loader, and hierarchies.loader.
Alternatively, you may load the library/all.loader file to load all library 
entities.

In this example we have several types of polygons that can be concentric.
This is represented by a concentric binary relation ensuring that whenever 
a polygon is moved towards a new position, all polygons likewise concentric 
are moved along with them.