9.1 Using the templates to write rule statements 241 9.1.2.2 Set functions A set function is any scalar numeric property of a set of data items. S5. < set function>::¼ {number | sum | total | maximum | minimum | average | mean | median | latest | earliest} 9.1.2.3 Comparison operators There are two types of comparison operators: inequality operators and equality operators. S6. < inequality operator>::¼ {{no | } {more | less | later | earlier} than | at {least | most} < literal> {more | later} than | {no | } {later | earlier} than < literal> {after | before}} Thus an inequality operator in a rule statement can be any of the following: 1. `more than', `less than', `later than', `earlier than', `no more than', `no less than', `no later than', `no earlier than'; 2. `at least < literal> more than', `at most < literal> more than', `at least < literal> later than', `at most literal> later than';