The following BNF defines the set of legal KIF rules. The last argument in a forward rule is called the consequent of the rule. Analogously, the first argument in a reverse rule is called the consequent . The following BNF defines the set of legal KIF definitions. KIF definitions can be complete in that they specify an expression that defines the concept completely, or they can be partial in that they constrain the concept without necessarily giving a complete equivalence.
Partial definitions can be either conservative or unrestricted .
Conservative definitions are restricted in that their addition to a knowledge base does not result in the logical entailment of any additional sentences not containing the constant being defined. A form in KIF is either a sentence, a rule, or a definition. A knowledge base is a finite set of forms.