<xs:schema xmlns:tns=“myns

targetNamespace="http://myns"
xmlns:xs="http://www.w3.org/2001/XMLSchema"

>

<xs:element name="a" />

<xs:complexType name="c" >
      <xs:sequence minOccurs="2" maxOccurs="4">
           <xs:sequence minOccurs="19" maxOccurs="20">
                <xs:element ref="tns:a" />
           </xs:sequence>
      </xs:sequence>
</xs:complexType>

<xs:element name="c" type="tns:c" />

</xs:schema>