Horn clauses

• Prolog cannot handle arbitrary logic over relations

• It supports Horn clauses only

  • p1 ? p2 ? ... ? pn ? q
  • p

• Implication is written “backwards”

• mother(M,C) :- parent(M,C), female(M).

• female(karen).

• Cannot say

  • p1 ? p2 ? q1 ? q2
  • And other stuff

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