<element name="addressBook" xmlns="http://relaxng.org/ns/structure/1.0">
<zeroOrMore>
<element name="card">
<element name="name">
<text/>
</element>
<element name="email">
<text/>
</element>
<optional>
<element name="prefersHTML">
<empty/>
</element>
</optional>
</element>
</zeroOrMore>
</element>