As the base type declared inside the type:

<xs:simpleType name="uri">
  <xs:restriction>
    <xs:simpleType>
      <xs:restriction base="xs:anyURI">
        <xs:pattern name="\w+:.*"/>
      </xs:restriction>
    </xs:simpleType>
    <xs:enumeration value="http://cde.berkeley.edu/">
    <xs:enumeration value="http://sims.berkeley.edu/">
  </xs:restriction>
</xs:simpleType>

Why you would ever want to do this...

...maybe it is there for completeness: "everywhere you can refer to a type you can give an explicit definition of that type instead"