KL-ONE and KL-TWO are programming systems for representing and manipulating knowledge. They have been in development at BBN during the late 1970s. KL-ONE is an outgrowth of the semantic net <227> school of representation, and includes some of the features of Minsky’s frames <77>. Classes are represented by “Concepts” (similar to frames). and their properties are represented by “roles” (similar to slots of frames). Concepts and roles are organized into separate taxonomies based on the relation of subsumption (similar to ISA) (see ISA hierarchy <108>). giving a notion of sub- and super-Concepts, as well as sub- and super- Roles. There is a system of inheritance by which Concepts (and Roles) acquire attributes of their super-Concepts (and super-Roles). and there is an algorithm, called classification, that discovers appropriate subsumption relationships that are not explicitly stated. A formal specification of the semantics of KL-ONE has been given [Sidner et al 83] in which one-place predicates are associated with Concepts and two-place predicates with Roles, where the predicates may be complex. A new system, called KL-TWO, includes KL-ONE plus a separate component for asserting facts about individuals. This component provides a propositional calculus with equality where Concepts and Roles may be used as one- and two-place predicates respectively.
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