we have that the predicates known and notknown mentioned in the table have a special meaning
as well: they express knowledge concerning the derivability of values for variables, which
again goes beyond the contents of the fact set and the rule base of a production system. We
call such predicates meta-predicates. In Section 3.2.1 we will turn again to the meanings of
the predicates and the effects the different actions have on a fact set.