7.2.2 Specifying Predicates

Object predicates (see Predicate System) are specified using the ‘@when’ attribute of certain nodes. It must contain a string of references understood by the system (see domain of discourse, Predicate System), all of which much match for the predicate to be true.

  <question id="describe" type="noyes"
            label="Any special notes for this location?" />

  <question id="vacant_desc" type="textarea"
            when="q:describe vacant property"
            label="Show only when a vacant property with the
                   question 'describe' non-empty and non-zero" />

In Figure 7.4 above, question ‘vacant_desc’ will be applicable when all of the values of ‘vacant’, ‘property’, and ‘q:describe’ are true.¹⁹ Within the context of the Program XML, this concretely means that the classifications ‘vacant’ and ‘property’ are true, and that the question ‘describe’ is answered “yes”. It reads as a sentence: “‘vacant_desc’” is applicable when we should  “describe a vacant property”.