17. The knowledge system as claimed in claim 16, wherein said set of predetermined conditions is specified by a predefined set of expressions which have values concluded by said rules, said step of said inference procedure is the step at which the value of an expression is found, and said conditions are found to hold when the expression having its value found is a member of said predefined set of expressions.
17. The knowledge system as claimed in claim 16, wherein said set of predetermined conditions is specified by a predefined set of expressions which have values concluded by said rules, said step of said inference procedure is the step at which the value of an expression is found, and said conditions are found to hold when the expression having its value found is a member of said predefined set of expressions. 18. The knowledge system as claimed in claim 17, wherein said evaluation system includes means for determining which rules concluded about the expression having its value found, and means for determining which factors in the rules caused said rules to conclude about the expression having its values found.