Same answer as yesterday :-)

Substution groups were specifically designed to reconstruct the use of
parameter entities for content models as mechanisms for controlled
extension.

The requirement that the type of a substiution group member be derived
from the type of its head imposes the condition you require.

Again, as yesterday, if you want to require restriction, then use
"final='extension'" on the base type.

Quick sketch:

Table schema doc:
<xs:schema targetNamespace="http://www.example.org/table"
           xmlns:tc="http://www.example.org/cell">

 <xs:import namespace="http://www.example.org/cell"/>

 . . .

 <xs:element name="cell">
  <xs:complexType>
   <xs:sequence>
    <xs:element ref="tc:content"/>
   </xs:sequence>
  </xs:complexType>
 </xs:element>

</xs:schema>

Table cell schema doc:
<xs:schema targetNamespace="http://www.example.org/cell"
           xmlns="http://www.example.org/cell">

 <xs:element name="content" abstract="true" type="contentType"/>
 
 <xs:complexType name="contentType">
  <xs:sequence>
   ...
  </xs:sequence>
 </xs:complexType>

</xs:schema>

User schema doc:
<xs:schema targetNamespace="http://www.example.org/user"
           xmlns:tc="http://www.example.org/cell"
           xmlns:tt="http://www.example.org/table"
           xmlns="http://www.example.org/user">
 
 <xs:import namespace="http://www.example.org/table"/>
 
 <xs:import namespace="http://www.example.org/cell"/>
 
 <xs:complexType name="myDisplay">
  <xs:sequence>
   ...
   <xs:element ref="tt:table"/>
   ...
  </xs:sequence>
 </xs:complexType>
 
 <xs:element name="mdContent" type="mdContentType"
             substitutionGroup="tc:content"/>
 
 <xs:complexType name="mdContentType">
  <xs:complexContent>
   <xs:restriction base="tc:contentType">
    ...
   </xs:restriction>
  </xs:complexContent>
 </xs:complexType>

</xs:schema>

Now an instance of the form

  <tt:table>
   . . .
   <tt:cell>
     <my:mdContent>
      . . .
     </my:mdContent>
   </tt:cell>
   . . .
  </tt:table>

is schema-valid.

Hope this helps,

ht
-- 
 Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@i...
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]

From easirois@r... Wed Mar 23 16:21:37 2005
Received: from bart.w3.org ([128.30.52.40])
	by frink.w3.org with esmtp (Exim 4.44)
	id