The ATOM spec has the concept of a "construct" which is much like a complex type in XML Schema.

Here is the "Person Construct" translated into XML Schema:

<xs:complexType name="PersonConstruct">
<xs:all>
<xs:element ref="atom:name"/>
<xs:element ref="atom:uri" minOccurs="0"/>
<xs:element ref="atom:email" minOccurs="0"/>
<xs:any ref="minOccurs="0" maxOccurs="unbounded" namespace="##other"/>
</xs:all>
<xs:attributeGroup ref="atom:common"/>
</xs:complexType>

An example:

<atom:author>
<atom:email>alex@milowski.com</atom:email>
<atom:name>Alex Milowski</atom:name>
<my:dog>Hudson</my:dog>
</atom:author>