<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

   <xs:element name="test">
      <xs:complexType>
	     <xs:sequence>
		    <xs:element name="x" type="X" />
			<xs:element name="y" type="Y" />		
		 </xs:sequence>
	  </xs:complexType>
   </xs:element>
   
   <xs:complexType name="X">
     <xs:group ref="List1" />
	 <xs:assert test="a and b and d">
	    <xs:annotation>
           <xs:documentation>
		       Elements a, b and d are mandatory.
		   </xs:documentation>
        </xs:annotation>
	 </xs:assert>
   </xs:complexType>
   
   <xs:complexType name="Y">
     <xs:group ref="List1" />
	 <xs:assert test="a and b and c and d">
	    <xs:annotation>
           <xs:documentation>
		       Elements a, b, c and d are mandatory.
		   </xs:documentation>
        </xs:annotation>
	 </xs:assert>
   </xs:complexType>
   
   <xs:group name="List1">
      <xs:sequence>
        <xs:element name="a" type="xs:string" minOccurs="0"/>
	    <xs:element name="b" type="xs:string" minOccurs="0"/>
	    <xs:element name="c" type="xs:string" minOccurs="0"/>
	    <xs:element name="d" type="xs:string" minOccurs="0"/>
	  </xs:sequence>
   </xs:group>
           
</xs:schema>