[PRD] Safeness definition in PRD (ACTION-826 complete)

********* NOTICE **********
My new email address at IBM is: csma@fr.ibm.com
My ILOG email address will not be forwarded after June 8
*****************************
all,

We all agree that the operational semantics of rules and rule sets in PRD 
requires some sort of safeness, at least that all the rule variables that 
occur in the action block must be bound in the condition.

To be able to specify the necessary notions, I added the definition of the 
sets of the unconditionally (UBV) and conditionally bindable variables 
(CBV), in atomic formulas [1], general condition formulas [2] and rules 
[3].

I use the definition of these sets to define safe condition formulas [4] 
(that can be evaluated relying on pattern matching only) and safe rules 
[5] (and rule sets).

The presentation of the definition of UBV and CBV could certainly be 
improved, but, more than anything, somebody must check that they are 
correct, and that they extend the definition of safeness in Core. *Jos* 
comes to mind. Bugs can be corrected later, but the definitions must be, 
at least, essentially correct.

[1] http://www.w3.org/2005/rules/wiki/PRD#def-bindable-var
[2] http://www.w3.org/2005/rules/wiki/PRD#def-bindable-var_formula
[3] http://www.w3.org/2005/rules/wiki/PRD#def-bindable-var_rule
[4] http://www.w3.org/2005/rules/wiki/PRD#def-safe-formula
[5] http://www.w3.org/2005/rules/wiki/PRD#def-safe-rule

This completes my ACTION-826.

Cheers,

Christian

ILOG, an IBM Company
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