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

<xs:element name="foo" />

<xs:complexType name="mybase">
  <xs:choice>
    <xs:element ref="x:foo" />
  </xs:choice>
</xs:complexType>

</xs:schema>