1<?xml version="1.0"?>
2<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
3  <xs:complexType name="A">
4    <xs:all>
5      <xs:element name="b" type="xs:string" minOccurs="0"/>
6    </xs:all>
7  </xs:complexType>
8
9  <xs:element name="a" type="A"/>
10</xs:schema>
11