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From: wagner@iaoobelix.UUCP
Newsgroups: comp.lang.prolog
Subject: Re: Prolog control structures - (nf)
Message-ID: <7000007@iaoobelix.UUCP>
Date: Thu, 2-Apr-87 03:42:00 EST
Article-I.D.: iaoobeli.7000007
Posted: Thu Apr  2 03:42:00 1987
Date-Received: Sat, 4-Apr-87 19:07:42 EST
References: <10588@topaz.UUCP>
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Nf-ID: #R:topaz:10588:iaoobelix:7000007:000:1039
Nf-From: iaoobelix!wagner    Apr  2 09:42:00 1987

/***** iaoobelix:comp.lang.pro / topaz!chomicki /  5:28 pm  Mar 31, 1987*/

>   In other words, xor(G1,G2) produces
>     -- all and only solutions of G1 if one exists, XOR
>     -- all and only solutions of G2 if one exists
...
> Jan Chomicki (chomicki@aramis.rutgers.edu)	Phone: (201) 932-3999
> Dept.of Computer Science, Rutgers University, New Brunswick, NJ 08903
> Usenet: {...harvard,pyramid,seismo,nike}!rutgers!aramis!chomicki
> Arpanet: chomicki@aramis.rutgers.edu

The above `xor' predicate seems to be identical with something called
`default(X,Y)' (if I remember it right) available in Prolog-II. `default(X,Y)'
behaves like X if there is at least one solution for X, otherwise it behaves
like Y.

There are some other Prologs implementing this (e.g I saw one on the
Macintosh).

Juergen Wagner,		      USENET: ...seismo!unido!iaoobel!wagner
("Gandalf")			or	  ...!pyramid!iaoobel!wagner

Mail:	Juergen Wagner
	Fraunhofer-Institut IAO
	Rosenbergstr. 28
	D-7000 Stuttgart 1
	Federal Republic of Germany
Phone:	+ 49 711 6648-205