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12.6 Designing the Concept Hierarchy 297

and one can attach notes to events. Notes are made by medical personnel.

The following are the content models:

<!--

A chart consists of one patient

and a sequence of events.

-->

<!ELEMENT Chart (Patient,Event*)>

<!--

An event is an admission, test, prescription

or operation. It also has at least one category

and may have any number of notes.

-->

<!ELEMENT Event

((Admission|Test|Prescription|Operation),

Category+, Note*)

>

An XML DTD is developed by carefully restating requirements and defi-

nitions using simple sentences to relate concepts to other concepts. These

restatements appear in the comments of the DTD above. Use only verbs

and verb phrases such asconsists of,contains,has,is,may,must,is a sequence

of,has exactly one,has at least oneandhas at most one. These expressions are

easily translated into content models. The content models can be con-

verted back into sentences which can be verified against the requirements

and definitions.

2. RDF.The concepts are organized according to subclass relationships. This
   is very different from the hierarchical organization of an XML document.
   In the medical chart ontology, we know that “a chart consists of one pa-
   tient and a sequence of events,” so that there are relationships between
   the chart concept and the patient and event concepts. However, neither
   of these relationships is a subclass relationship. We also know that “an
   event is an admission, test, prescription, or operation,” so there are re-
   lationships between these concepts. Unlike the relationships with chart,
   these four relationships do represent subclass relationships. How can one
   determine that a relationship is a subclass relationship?