subtyping

Dear XML Schema editor,

Observation:
In doing a comparison between the EXPRESS data modeling language (ISO
10303-11) and XML Schema, I noticed that in EXPRESS it is possible to
create a subtype that both is a restriction (e.g., attribute value range
restriction) of its supertype and an extension (e.g., an addition of
another attribute) of its supertype. From the expressions for simpleContent
complexContent (see below) it appears that in order to do the same thing in
XML Schema that in intermediate throw away subtype would need to be
created.

<simpleContent
  id = ID
  {any attributes with non-schema namespace . . .}>
  Content: (annotation? , (restriction | extension))
</simpleContent>


<complexContent
  id = ID
  mixed = boolean
  {any attributes with non-schema namespace . . .}>
  Content: (annotation? , (restriction | extension))
</complexContent>


Question:
Would it be worthwhile to permit both restriction and extension to be in
the same simpleContent and complexContent.


Sincerely,

Mike McCaleb