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Semantics 1: Stratified Datalog
•Split datalog program into
“strata”. Each stratum may use
negation of IDBs defined by previous strata.
Evaluate program in stages
•Example: find pairs x,y that are not connected:
T(x,y) :-
R(x,y)
T(x,y) :- R(x,z), T(z,y)
Answer(x,y) :- : T(x,y)
T(x,y) :- R(x,z), T(z,y)
Answer(x,y) :- : T(x,y)
here we have 2 strata