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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: List of Folk Theorems
Message-ID: <6473@goanna.cs.rmit.oz.au>
Date: 24 Jun 91 08:52:13 GMT
Article-I.D.: goanna.6473
References: <3470@geocub.UUCP>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 38

In article <3470@geocub.UUCP>, billaud@geocub.UUCP (Michel BILLAUD)
QUOTES

> What about:
> 
> 5. If b is deterministic and followed by a !, you can switch them:
>      p :- a,...,b,!,c,...     
>      p :- a,...,!,b,c,...

This is false.  Consider
	p :- fail, !.				p :- !, fail.
	p :- write(yoo-hoo).			p :- write(yoo-hoo).
	?- p.					?- p.
    =>	yoo-hoo				    =>	no
If b is determinate (not "deterministic") and cannot fail, _then_ you
can switch them.


> From csa09@seq1.keele.ac.uk Mon May 27 12:04:26 1991
> From: "P." Singleton <csa09@seq1.keele.ac.uk>

> If a (deterministic) goal doesn't instantiate its arguments 
> any further (perhaps it has useful side-effects), then you 
> can replace it by
> 
> 	\+ \+ goal

The DEC-10 Prolog library defines gcc(G) :- \+ \+ X.
It stands for "garbage collecting call".

> Nota : \+ = not

Note:  in DEC-10 Prolog and Quintus Prolog 'not' is _not_ the same
as "unprovable" \+ .  In both Prologs, 'not' is not an evaluable
predicate, but is defined in the library, and is defined to report
an error when it won't be sound.  Stick with \+ for portability.
-- 
I agree with Jim Giles about many of the deficiencies of present UNIX.