[PRD] Consensus of the PRD TF regarding refraction, conflict resolution and Modify

All,

The PRD task force agreed, today, that the PRD spec could be repaired with 
the following changes :
- for the purpose of refraction and recency, the states of the fact base 
after each atomic action must be taken into account, not only after each 
action block. That is, all the states of the fact base must be taken into 
account, not only the ones in which the conflict resolution strategy is 
applied. This can be considered the correction of a bug, since all the PR 
engines that we want to cover work that way;
- the facts that Modify is not an atomic action, but a compound one, must 
be made explicit (currently, the spec says that it is atomic, but it 
follows from the formal definition in conjunction with taking intermediate 
steps into account, that it is not. Which is correct, since some of the PR 
engines that we want to follow do not have an atomic Modify action);
- to make sure that all the cases are taken into account, rule instance 
must be defined with respect to the rules normalized in CLP form, that is, 
, with all the constants in the condition formula being replaced with 
universally quantified variables and the conjunction of a constraint 
equating the variable to the constant (one corner case that is not 
correctly taken into account with the current definition and that will be 
with the more precise one, for instance, is the case where the condition 
contains an disjunction of ground facts: "if ... (P(a) or Q(b)... 
then...", will be correctly processed under he current specification if 
normalized as: "forall ?x, if ...(  (p(a) and ?x=a) or (q(?x) and ?x=b) 
... then ..." - with some caveat wrt safeness).

More over, the PRD TFwould like to expose a new atomic action in the 
specifition, namely: RetractAll(frame) that retracts all the values of the 
"frame"'s attribute for the "frmae"'s object. The atomic action would be 
useful to make the semantics of Modify more easily intellegible (even if 
not exposed), and its exposition would make some implementations easier 
without harming others. However, the TF will not insist RetractAll being 
exposed if the cost was a 2nd last call.

Cheers,

Christian

IBM
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