Handling Irregular Data Hierarchies
One of the benefits of using XML is that it allows you to model irregular data hierarchies, including data with the following characteristics.
Collections of heterogeneous elements
Structures with many optional elements
Structures where the order is not apparent
Recursive structures
Structures with complex containment requirements
The pole.xml example in this section contains heterogeneous collections of elements, with a complex set of containment requirements. A <section> element can contain an arbitrary collection of <title> elements, <p> elements, <list> elements, and so on. Many elements are indeed optional: A <section> does not have to contain <p> or <list> elements, or other <section> elements. It is important to preserve the order of most elements in the output — that is, the first <section> comes before the second <section>. The structure is recursive because a <section> element can contain other <section> elements. The <emph> element is allowed anywhere. For example, you might add it to the <title> element, as follows:
<title>To the Pole... <emph>and Back!</emph></title>
The mechanism for handling data-driven transformations is similar to event-handling in traditional programming languages. Template rules or fragments are analogous to event handlers, which are called whenever the specified events occur. An element matched in the source document triggers a call to the specified template rule or fragment. A template rule or fragment can be called either by its name or according to the matched element type.