<!ELEMENT address      - -  ((street, pobox?, city, state,
                       zip) | abc | sr+)                 >

<xs:element name="address">
<xs:complexType>
  <xs:choice>
    <xs:sequence>
      <xs:element ref="street"/>
      <xs:element ref="pobox" minOccurs="0" maxOccurs="1"/>
      <xs:element ref="city"/>
      <xs:element ref="state"/>
      <xs:element ref="zip"/>
    </xs:sequence>
    <xs:element ref="abc"/>
    <xs:element ref="sr" minOccurs="1" maxOccurs="unbounded"/>
  </xs:choice>
</xs:complexType>