hi all,   
   default logic is a prominent example of a nonmonotonic logic.  a   
   default theory is usually a pair (W, D) where W is a set of formulae   
   and D a set of default rules. in most systems, the 'new information'   
   that's being learned that leads to the nonmonotonicity of the system is   
      
   always formulae added to or retracted from W.  does anyone know of   
   studies of variants of this logic where D is changed? that is where one   
      
   can retract and add new default rules?   
   thanks. -per   
      
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