XML Schema Question (Redefinig Elements)

Hi,

I have a short question I have not been able to answer reading the XSD specs.

Suppose I have a schema A defining an element Foo and a schema B that includes A. Is it ok for schema B to also define Foo (in a different way)? Does such a redefinition take precedence over A's definition of Foo? Is there a constraint that every valid Foo(B) must also be a valid Foo(A)?

[I am NOT intending to use/cannot use the <redefine> element, BTW]

Thanks in advance for any help.

Jan