<?xml version=“1.0”?> <xs:schema xmlns:xs=“www.w3.org/2001/XMLSchema”>

<xs:complexType name="A">
  <xs:all>
    <xs:element name="b" type="xs:string" minOccurs="0"/>
  </xs:all>
</xs:complexType>

<xs:element name="a" type="A"/>

</xs:schema>