13.3 Atoms: Their Types and Weights 249 Note that SubsetLink refers to a purely extensional subset relationship, and that Inheri- tanceLInk should be used for the generic "intensional + extensional" analogue of this — more on this below. SubsetLink could more consistently (with other link types) be named Extension- allnheritanceLink, but SubsetLink is used because it’s shorter and more intuitive. There are links representing Boolean operations AND, OR and NOT. For instance, we may say ImplicationLink ANDLink ConceptNode young ConceptNode beautiful ConceptNode attractive or, using links and VariableNodes instead of ConceptNodes, AverageLink $X ImplicationLink ANDLink EvaluationLink young $X EvaluationLink beautiful $x EvaluationLink attractive $X NOTLink is a unary link, so e.g. we might say AverageLink $X ImplicationLink ANDLink EvaluationLink young $X EvaluationLink beautiful $x EvaluationLink NOT EvaluationLink poor $X EvaluationLink attractive $X ContextLink allows explicit contextualization of knowledge, which is used in PLN, e.g. ContextLink ConceptNode golf TInheritanceLink ObjectNode BenGoertzel ConceptNode incompetent says that Ben Goertzel is incompetent in the context of golf. 13.3.2 Variable Atoms We have already introduced VariableNodes above; it’s also possible to specify the type of a VariableNode via linking it to a VariableTypeNode via a TypedVariableLink, e.g. VariableTypeLink VariableNode $X VariableTypeNode ConceptNode which specifies that the variable $X should be filled with a ConceptNode. Variables are handled via quantifiers; the default quantifier being the AverageLink, so that the default interpretation of HOUSE_OVERSIGHT_013165