Subject: Re: not with trace or not with fail ???

From: Jan Wielemaker <[email-masked]>
Date: Mon, 11 Dec 2000 10:16:33 +0100

On Mon, 27 Nov 2000, jean-F. Dazy wrote:
>?- not(v).
>ERROR: Undefined procedure: v/0
>
>
>?- unknown(_ , fail).
>Yes
>?- not(v).
>
>Yes
>
>it is dangerous  !
>2 differents semantics ????

Yes, the unknown flag (actually implemented through set_prolog_flag/2
these days) affects semantics.  It should be threated very carefully. 
Basicaly, only use it to run large complex programs written for old
Prolog systems for which this was the default or locally in a module
to take calculated advantage of the changed semantics.

Please note that with the unknown flag active, autoloading doesn't
work for that indicated module, so all required library predicates
must be impored explicitely.

	Cheers --- Jan