Allow key for complexType with simpleContent

It would be helpful in some cases to be able to apply a key to elements 
that are defined as a complexType with simpleContent.

Currently, Clause 3 of  Validation Rule Identity-constraint Satisfied 
(http://www.w3.org/TR/xmlschema-1/#cvc-identity-constraint) states:
For each node in the ·target node set· all of the {fields}, with
that node as the context node, evaluate to either an empty node-set
or a node-set with exactly one member, **which must have a simple
type**. (my emphasis). [thanks to Jeni Tennison for pointing this out to me]

The only implementation I have come across that has NOT allowed identity 
constraints on elements of complexType/simpleContent is MSXML 4.0.

Thanks

Rick Taylor
XML Developer
PPDM Association