<xs:schema xmlns:tns="http://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>