Cyclic definitions allowed ?

I have validated the following document in XML Authority and it is happy.

But Oracle 9iR2 gives me the following error when I try to register the schema below : "ORA-31151: Cyclic definition encountered for element 'span' ".
Is this normal ?  I would very much appreciate if someone could then point me to a section in the spec that allows or prohibits these kinds of definitions.

P. Andries

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<xsd:schema xmlns:xsd = "http://www.w3.org/2001/XMLSchema" elementFormDefault = "qualified">

<xsd:group name = "liContentGroup">
  <xsd:choice>
   <xsd:element ref = "span"/>
   <xsd:element ref = "b"/>
   <xsd:element ref = "i"/>
   <xsd:element ref = "ol"/>
   <xsd:element ref = "ul"/>
  </xsd:choice>
 </xsd:group>

<xsd:element name="span" type="spanType"/>
<xsd:element name="b" type="spanType"/>
<xsd:element name="i" type="spanType"/>
<xsd:element name="u" type="spanType"/>
<xsd:element name="ol" type="olType"/>
<xsd:element name="ul" type="ulType"/>
<xsd:element name="li" type="liType"/>

<xsd:complexType name = "spanType" mixed = "true">
    <xsd:choice minOccurs = "0" maxOccurs = "unbounded">
        <xsd:element ref = "span"/>
        <xsd:element ref = "b"/>
        <xsd:element ref = "i"/>
        <xsd:element ref = "u"/>
    </xsd:choice>
    <xsd:attribute name = "style" type = "xsd:string"/>
</xsd:complexType>

<xsd:complexType name = "olType">
    <xsd:sequence minOccurs = "0" maxOccurs = "unbounded">
           <xsd:element ref = "li"/>
    </xsd:sequence>
    <xsd:attribute name = "type" type = "xsd:string"/>
 </xsd:complexType>
 
  <xsd:complexType name = "ulType">
      <xsd:sequence minOccurs = "0" maxOccurs = "unbounded">
           <xsd:element ref = "li"/>
      </xsd:sequence>
 </xsd:complexType>
 
  <xsd:complexType name = "liType" mixed = "true">
      <xsd:sequence>
           <xsd:choice minOccurs = "0" maxOccurs = "unbounded">
                <xsd:group ref = "liContentGroup"/>
           </xsd:choice>
      </xsd:sequence>
 </xsd:complexType>
 </xsd:schema>