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Logic Programming - Negation

  • As soon as negation is allowed, cycles in a dependency graph become problematic.
      • Example: What is the meaning of win(x) :- not win(x) ?

  • A Solution: Stratification
    • Mark edges with negation in the dependency graph
    • Separate predicates which are connected through a positive edge in a individual stratum
    • Strata can be (partially) ordered
    • If each predicate occurs only in one stratum, then the program is called stratifiable
    • Each stratum can be evaluated as usual and independently from other strata

      → This guarantees a unique interpretation of a Logic Program using negation


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