XML Schema: Extension and Restriction from a Mixed Complex Type Base Typ

["Paul Hermans" <paul.hermans@amplexor.com>]

I need to start and derive from following datatype.

<xs:complexType name="SimpleLiteral">
    <xs:annotation>
      <xs:documentation xml:lang="en"> This is the default type. It
        permits text content only with optional xml:lang attribute. 
        Text is allowed because
        mixed="true", but sub-elements are disallowed because minOccurs="0" and maxOccurs="0" are on
        the xs:any tag. 
        This complexType allows for restriction or extension permitting child
        elements. </xs:documentation>
    </xs:annotation>
    <xs:complexContent mixed="true">
      <xs:restriction base="xs:anyType">
        <xs:sequence>
          <xs:any processContents="lax" minOccurs="0" maxOccurs="0"/>
        </xs:sequence>
        <xs:attribute ref="xml:lang" use="optional"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
  
  This is fine for instance elements such as:
  <a xml:lang="en">abcdefg</a>
  
  But in one case I need to have following model:
  <a>
    <b>2008-12-12</b>
    <c>2008-12-14</c> <!-- optional -->
  </a>
  
  In a normal world modeled as:
  <xs:complexType name="Temporal">
    <xs:sequence>
      <xs:element name="a" type="xs:date"/>
      <xs:element name="b" type="xs:date" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>
  
  Now my derivation approach:
  1) First extending by allowing elements
  <xs:complexType name="SimpleLiteralExtended">
    <xs:complexContent mixed="true">
      <xs:extension base="SimpleLiteral">
        <xs:sequence>
          <xs:any processContents="lax" minOccurs="0" maxOccurs="unbounded"/>
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  
  So far, so good.
  
  2) Restricting, getting rid of mixed, of the xml:lang attribute, and being more specific for the elements.
  <xs:complexType name="SimpleLiteralExtendedRestricted">
    <xs:complexContent>
      <xs:restriction base="SimpleLiteralExtended">
        <xs:sequence>
          <xs:element name="a" type="xs:date"/>
          <xs:element name="b" type="xs:date" minOccurs="0"/>
        </xs:sequence>
        <xs:attribute ref="xml:lang" use="prohibited"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
  
  In Xerces-Java this leads to following error:
  "E [Xerces] derivation-ok-restriction.5.4.2: Error for type 'SimpleLiteralExtendedRestricted'.  
  The particle of the type is not a valid restriction of the particle of the base."
  
  If I comment out the second line, the error disappears.
  <xs:sequence>
          <xs:element name="a" type="xs:date"/>
          <!-- <xs:element name="b" type="xs:date" minOccurs="0"/> -->
  </xs:sequence>
  
  Can someone explain this behavior or is it a Xerces issue?
  
  The same works without problems in Saxon-SA.
  
  
  Paul