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<?xml version="1.0"?>
<xs:schema xmlns:xs="http://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>