<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="foo">
    <xs:complexType>
        <xs:sequence minOccurs="0"  maxOccurs="unbounded">
            <xs:element name="a" type="xs:string" minOccurs="0"  />
            <xs:element name="b" type="xs:string"/>
            <xs:element name="c" type="xs:string" minOccurs="0" />
        </xs:sequence>
    </xs:complexType>
</xs:element>
</xs:schema>