grey skin tail 1 1 trunk legs 4 isa e1 name clyde elephant
Self-Instructional Material 119 Weak Slot and Filler Structures NOTES Using inheritance • To find the value of a property of e1 , first look at e1 . • If the property is not attached to that node, ‘climb’ the isa link to the node’s parent and search there. ■ isa signifies set membership: ■ ako signifies the subset relation: • Repeat, using isa/ako links, until the property is found or the inheritance hierarchy is exhausted. • Sets of things in a semantic network are termed types . • Individual objects in a semantic network are termed instances . Examples of Semantic Networks State: I own a tan leather chair. furniture ako person chair seal isa isa me owner my-chair colour tan covering leather brown part isa Event: John gives the book to Mary. give isa John agent event7 object book23 isa beneficiary isa person isa Mary book Complex event: Bilbo finds the magic ring in Gollum’s cave. Bilbo isa habbit ako person agent event9 object magicring2 isa ring isa location owned-by Gollum find cave77 isa cave
Self-Instructional 120 Material Weak Slot and Filler Structures NOTES 5. 2.3 Extending Semantic Nets Here, we will consider some extensions to Semantic nets that overcome a few problems (see Questions and Exercises) or extend their expression of knowledge. Partitioned Networks Partitioned Semantic Networks allow for: • propositions to be made without commitment to truth. • expressions to be quantified. Basic idea: Break network into spaces which consist of groups of nodes and arcs and regard each space as a node. Consider the following: John believes that the earth is flat. We can encode the proposition the earth is flat in a space and within it have nodes and arcs that represent the fact (see Fig. 5.7). We can have the nodes and arcs to link this space with the rest of the network to represent John’s belief. believes instance agent John event1 object space 1 earth flat instance insurance Object 1 Prop 1 has_property Fig. 5.7 Partitioned Network Now consider the quantified expression: Every parent loves their child. To represent this we: • Create a general statement , GS, special class. • Make node g an instance of GS. • Every element will have at least 2 attributes: ■ a form that states which relation is being asserted. ■ one or more forall ( ∀ ) or exists ( ∀∃ ) connections—these represent universally quantifiable variables in such statements e.g. x , y in ∀ x parent(x) → ∀∃ y : child(y) ∧ loves(x, y) Here we have to construct two spaces one for each x , y . Note: We can express variables as existentially qualified variables and express the event of love having an agent p and receiver b for every parent p which could simplify the network.
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