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Logtalk - Object oriented extension to Prolog
Release 2.23.1

Copyright (c) 1998-2005 Paulo Moura.  All Rights Reserved.
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To load this example and for sample queries, please see the SCRIPT 
file.

This example illustrates some variants of the "diamond problem" 
(multi-inheritance conflicts and ambiguities) and its respective 
solutions on Logtalk.

This classical problem can be simply described by constructing a 
"diamond" of objects and inheritance links as follows:

        A       -- contains default definition for a predicate m/0
	  /   \
    B       C   -- contains redefinitions of the predicate m/0
      \   /
        D       -- inherits both redefinitions of the predicate m/0

As such, the object D inherits two conflicting definitions for the 
predicate m/0, one from object B and one from object C. If we send 
the message m/0 to object D, is ambiguous which inherited definition 
should be used to answer it. Depending on the nature of the objects 
A, B, C, and D, the correct answer can be the redefinition of m/0 in 
object B, the redefinition m/0 in object C, or both redefinitions.
A programming language supporting multi-inheritance should provide 
programming mechanisms allowing easy implementation of each possible 
solution.

Note that, in the context of Logtalk, the diamond problem may occur with 
prototype hierarchies, class hierarchies, protocol hierarchies, or when 
using category composition.

This example deals with three variants of the diamond problem, illustrated 
using prototype hierarchies:

	diamond1
		illustrates the inherited definition which is visible due to the
		Logtalk predicate lookup algorithm
	diamond2
		presents a solution for making the overridden inherited definition 
		the visible one
	diamond3
		presents a solution which allows both inherited definitions to be 
		used in D