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

<xs:import namespace="urn:foo" schemaLocation="import-subst-dbr-base.xsd" />

<xs:element name="bar" substitutionGroup="x:foo" />

<xs:complexType name="myderived">
  <xs:complexContent>
    <xs:restriction base="x:mybase">
      <xs:choice>
        <xs:element ref="y:bar" />
      </xs:choice>
    </xs:restriction>
  </xs:complexContent>
</xs:complexType>

</xs:schema>