XML Schema -> DTD?

Wasn't it a design goal to allow all XML Schema definitions to be
translated
to (less restrictive) DTD definitions?

So what is the equivalent DTD for the following schema?

<xsd:element name="foo">
  <xsd:complexType>
    <xsd:element name="myTag" type="a"/>
  </xsd:complexType>
</xsd:element>

<xsd:element name="bar">
  <xsd:complexType>
    <xsd:element name="myTag" type="b"/>
  </xsd:complexType>
</xsd:element>

<xsd:complexType name="a">
  <xsd:attribute name="key" type="xsd:IDREF"/>
</xsd:complexType>

<xsd:complexType name="b">
  <xsd:attribute name="key" type="xsd:ID"/>
</xsd:complexType>