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    ELECTRONIC NEWSLETTER ON  REASONING ABOUT ACTIONS AND CHANGE        
Issue 97011             Editor: Erik Sandewall            24.10.1997
Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/
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                    *********  TODAY  *********

Today's newsletter contains a question to the authors of the article
by Kakas and Miller that was received a week ago. It is an example
of how the Newsletter serves one of the key ideas in ETAI: that of
exposing submitted articles to discussion in our research community
*before* it is sent to the referees. The following is the standard
format for such debate contributions which will be used also in the
sequel.


              *********  ETAI PUBLICATIONS  *********

            ---  DISCUSSION ABOUT RECEIVED ARTICLES  ---

The following debate contributions (questions, answers, or comments)
have been received for articles that have been received by the ETAI and
which are presently subject of discussion. To see the full context,
for example, to see the question that a given answer refers to, or to
see the article itself or its summary, please use the web-page version 
of this Newsletter.

--------------------------------------------------------
|  Antonis Kakas and Rob Miller
|  Reasoning about Actions, Narratives and Ramification
--------------------------------------------------------

FROM: Michael Thielscher

Antonis and Rob,

I have a question concerning the notion of initiation and termination
points in case ramifications are involved. If my understanding of
your Definition 14 is correct, then there seems to be a problem with
undesired mutual justification. Take, as an example, the two
r-propositions

    dead  whenever  -alive
  -alive  whenever  dead

Suppose there are no other propositions, in particular no events, then

  H(0)={alive,-dead}  H(1)={-alive,dead}

seems to satisfy all conditions for being a model. The two uncaused
changes justify each other: 0 is an initiation point for dead since
0 is a termination point for alive, and vice versa.

Finding some least fixpoint, which you mention after the definition,
seems therefore vital for the correctness of the definition itself.
However, the corresponding operator must not have an interpretation
as argument. So I would think that instead of defining the notions of
"initiation and termination points for F in H relative to D" one
should define "initiation and termination points for F relative to D,"
that is, without reference to some H.


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