I have the following XSD which I'm generating JAXB bindings from... (only left relevant portions for this question):
<?xml version="1.0"?>
<xsd:schema 
          xmlns:xsd="http://www.w3.org/2001/XMLSchema"
          xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
          xmlns:simplify="http://jaxb2-commons.dev.java.net/basic/simplify"
          jaxb:extensionBindingPrefixes="simplify">...
 <xsd:element name="trace">
     <xsd:annotation>
        <xsd:documentation>
            containing screen navigation data
        </xsd:documentation>
    </xsd:annotation>
    <xsd:complexType>
        <xsd:choice minOccurs ="0" maxOccurs ="unbounded">
            <xsd:element ref="time"  />
            <xsd:element ref="event" />
            <xsd:element ref="location" />
            <xsd:element minOccurs="0" ref="index" ></xsd:element>
            <xsd:element minOccurs="0" ref="caseId"/>
            <xsd:element minOccurs="0" ref="contactAttId" />
            <xsd:element minOccurs="0" ref="action" />
        </xsd:choice>
    </xsd:complexType>
</xsd:element>