254 13 Local, Global and Glocal Knowledge Representation The asymmetric HebbianLink indicates that when the first argument is present in a situation, the second is also often present. The symmetric (default) version indicates that this relationship holds in both directions. The inverse versions indicate the negative relationship: e.g. when one argument is present in a situation, the other argument is often not present. 13.3.6 Procedure Nodes There are nodes representing various sorts of procedures; these are kinds of ProcedureNode, e.g. e SchemaNode, indicating any procedure e GroundedSchemaNode, indicating any procedure associated in the system with a Combo program or C++ function allowing the procedure to be executed e PredicateNode, indicating any predicate that associates a list of arguments with an output truth value e GroundedPredicateNode, indicating a predicate associated in the system with a Combo program or C++ function allowing the predicate’s truth value to be evaluated on a given specific list of arguments ExecutionLinks and EvaluationLinks record the activity of SchemaNodes and PredicateN- odes. We have seen many examples of EvaluationLinks in the above. Example ExecutionLinks would be: ExecutionLink step\_forward ExecutionLink step\_forward 5 ExecutionLink + ListLink NumberNode: 2 NumberNode: 3 The first example indicates that the schema "step forward" has been executed. The second example indicates that it has been executed with an argument of "5" (meaning, perhaps, that 5 steps forward have been attempted). The last example indicates that the "++" schema has been executed on the argument list (2,3), presumably resulting in an output of 5. The output of a schema execution may be indicated using an ExecutionOutputLink, e.g. ExecutionOutput Link + ListLink NumberNode: 2 NumberNode: 3 refers to the value "5" (as a NumberNode). 13.3.7 Links for Special External Data Types Finally, there are also Atom types referring to specific types of data important to