164 CHAPTER 7 How to write quality natural language rule statements 7.2.2 Definitional rule statements A definitional rule statement is a statement of a definitional rule. Every definitional rule statement in this book--and every definitional rule statement you write if you follow the method outlined in this book--has the following general formulation: T6. {A | An | The | } < definitional rule statement subject> {< qualifying clause> | } < verb phrase> by definition < definition>. Again, this template is too general for easy use in creating rule statements that correctly express each type of definitional rule. Section 9.2 in Chapter 9 lists specific templates for each type of definitional rule, along with more specific restrictions on suitable phrasing of subjects and definitions. 7.2.3 The subject of a rule statement With very few exceptions, every rule statement involves more than one term, of which only one can be the subject of that statement. For each type of rule statement, there are restrictions on which term can be the subject. For definitional rule statements, these restrictions vary depending on the type of rule, as described in Section 7.2.3.1. In any operative rule statement, the subject must be the term signifying the set of objects to be tested by the rule, as described in Sections 7.2.3.2­7.2.3.4. The discussion in Chapter 9 of the template(s) for each type of rule statement includes advice as to what the subject term (and any other terms) in the rule statement should be or not be. Unfortunately, I often encounter operative rule statements that violate this principle, produced by practitioners new to business rules, and even sometimes by experienced practitioners. For example a recent paper on business rules included the following rule statement: R179. { A `flammable' sticker must be displayed on a tank containing a combustible fuel. Since the subject of that rule statement is `flammable' sticker, the statement governs what is to be done with `flammable' stickers (rather than tanks containing combustible fuel). Thus, if I have a `flammable' sticker in my hand rather than on a tank, that sticker does not comply with the rule statement as written. If, however, I recast R179 as in R180, it is tanks rather than stickers which must be checked for compliance with the rule statement, and one can have a supply of spare stickers on hand. R180. A tank that contains a combustible fuel must be marked with a `flammable' sticker. Other rules that are easy to express in a way that violates this principle are those governing activities that depend on preceding activities having been completed. If an activity is permitted to occur only after some other activity has been completed, the subject of the rule statement must be the term that signifies the later activity rather than the earlier. Consider rule statements R181 and R182. The intention of each of these rule statements is to establish whether a passenger is permitted to board a flight. It is not a constraint on passengers checking in.