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>