RELAX NG directly supports four kinds of occurrence constraints on nodes: they may appear as exactly once (the default), optional, zero or more, or one or more. These are the most common cases in document design. If applications need a finer level of control, that can be achieved by using or combining these four basic occurrence constraints. If, for instance, you need to define that each book's description should have between two and six character elements, you can write the definition as two mandatory characters followed by four optional ones:

<!-- 1 -->
<element name="character">
 <attribute name="id"/>
 <element name="name">
  <text/>
 </element>
 <optional>
  <element name="born">
   <text/>
  </element>
 </optional>
 <element name="qualification">
  <text/>
 </element>
</element>
<!-- 2 -->
<element name="character">
  .../...
</element>
<!-- 3 -->
<optional>
 <element name="character">
 .../...
  </element>
 </element>
</optional>
<!-- 4 -->
<optional>
 <element name="character">
 .../...
 </element>
</optional>
<!-- 5 -->
<optional>
 <element name="character">
 .../...
 </element>
</optional>
<!-- 6 -->
<optional>
 <element name="character">
 .../...
 </element>
</optional>

This is certainly verbose, but in later chapters, you will see how to define and reuse patterns to reduce verbosity.

	Tip
	W3C XML Schema offers much more control over how many times an element may appear, but this degree of control creates a number of processing complexities. While RELAX NG's approach may, in this respect, seem less powerful, it compensates by imposing far fewer costs.

Figure 3-1 shows the schema and the instance document side by side. Even though information has been added to the schema that describes the content of the text nodes and the number of their occurrences, the schema keeps the same hierarchical structure as the instance document.

Figure 3-1. Comparing the Russian doll schema structure with that of the instance document

A schema in which different definitions are embedded in each other, like this one, in which the definition of the library element physically contains the definition of the author element that physically contains the definition of the name element—is often called a Russian doll schema after the nested matruschka dolls. In Chapter 5, you'll see how Russian doll schemas can be broken into independent patterns and then combined to reproduce the structure of the instance document. First, we'll examine the equivalent compact syntax for RELAX NG in the next chapter.