customizing enumeration vs. choice elements

[vihrao]

I have an XML element that lists administrator roles

<xs:simpleType name="administratorType">
<xs:restriction base="xs:string">
<xs:enumeration value="Application_admin" />
<xs:enumeration value="Database_admin" />
<xs:enumeration value="Server_admin" />
<xs:enumeration value="Desktop_admin" />
</xs:restriction>
</xs:simpleType>

I want to express one role to implicitly assume one or more other roles
As an example:
Database_admin to include the roles of Database_admin and Application_admin
Server_admin to include the roles of Server_admin, Application_admin, and Desktop_admin

I thought of expressing administratorType as complextype with abstract attribute and choice elements. But I don't know how this approach will help either.

How can I do this? I would appreciate any ideas