From: Ian Horrocks (horrocks@cs.man.ac.uk)
Date: 11/05/03
On November 4, Benjamin Grosof writes: > At 07:19 PM 11/4/2003 -0500, Sandro Hawke wrote: > > > > Remove the condition that a consequent is a conjunction. Its never a > > > conjunction, except as syntactic sugar for a conjunction of rules > > > each with a singleton-disjunction consequent. > > > >It needs to be a conjunction to get Horn expressivity using > >existentials scoped inside the consequent, right? > > no that's not really the issue, it's much simpler -- just a choice of the > syntactic convention for conjunction of atoms in the head. > We mainly need to define the truth value of an empty head as false rather > than true. The simplest way to define that would > be to permit the head only to be either a single atom or empty, as Pat > suggests. Or one could more permissively define it to be: a conjunction of > one or more atoms, or empty (interpreted as false), which is basically what > the mid-Oct. draft intended. To reiterate, the mid-Oct draft didn't "intend" this, it stated it unambiguously. Of course we may want to change the presentation to do a better job of explaining why the current semantics say what they say (or we may even want to change the semantics). Ian > > An atom here could be a variable-ized OWL-DL complex class expression, in > the extension to permit those. > B > > > -- sandro > > ________________________________________________________________________________________________ > Prof. Benjamin Grosof > Web Technologies for E-Commerce, Business Policies, E-Contracting, Rules, > XML, Agents, Semantic Web Services > MIT Sloan School of Management, Information Technology group > http://ebusiness.mit.edu/bgrosof or http://www.mit.edu/~bgrosof
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