The schema for XML Schema makes use of model groups.

This group defines top-level elements:

<xs:group name="schemaTop">
    <xs:choice>
      <xs:group ref="xs:redefinable"/>
      <xs:element ref="xs:element"/>
      <xs:element ref="xs:attribute"/>
      <xs:element ref="xs:notation"/>
    </xs:choice>
</xs:group>

which refers to:

<xs:group name="redefinable">
    <xs:choice>
      <xs:element ref="xs:simpleType"/>
      <xs:element ref="xs:complexType"/>
      <xs:element ref="xs:group"/>
      <xs:element ref="xs:attributeGroup"/>
    </xs:choice>
</xs:group>