Inheritance

Hello everyone,
Is the following possible with XML-schemas:

In Schema A I define a structure (element construction) and 2 kinds of
attributes: a kind that can not be redefined (Final) and one that can be
redifined (Overridable)

Now Schema B must take everything from A, structure and attributes.  It may
extend the structure with new elements, and add new attributes to elements
defined in A.  It may redefine overridable attributes from A (Polymorhing),
but it may not redefine final attributes.

I made a visual representation of this on
http://users.pandora.be/gdl/Schemas/Schema.gif .

Will I have to draw up a Slim version of XML-Schema to validate type B
schemas ?

tia
Tim