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% From: jssloka@monet.uwaterloo.ca (Scott Sloka)
% Subject: Help: making a binary predicate reversibly equal
% 
% Howdy.  My name is Scott Sloka and I am an AI enthusiast at the University of
% Waterloo.

Howdy.

%            I have a list of facts such as: nextto(a,b) and I would like
% nextto(a,b) to be equivalent to nextto(b,a).  This does not work:
% 
% nextto(X,Y):-nextto(Y,X).
% 
% because an endless loop is set up.  I am just beginning Prolog programming
% because I am doing a project on abstraction and I would like to include this
% parameter switch equalization in my routines for some of my predicates.  Any
% pointers would be greatly appreciated.  Thanks in advance.

This is a real common problem for beginners.  You need _two_ predicates, one
to handle the symmetry of the relation, and the other to provide the basic
facts.  Try something like:

	nextto( X, Y ) :- nextto_fact( X, Y ).
	nextto( X, Y ) :- nextto_fact( Y, X ).

	nextto_fact( a, b ).
	% and whatever other 'facts' are involved.

%                 -Scott
	-- Scot
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