ID attribute

Dear all,

I propose to relax the constraint of having at most one ID attribute at
complex types. Currently it is already allowed to have a complex type with 
simple
content that has an ID attribute and also it's simple content of type ID. In
other words elements in instance documents can already have 2 ids. Therefore
it would make sense to allow an arbitrary number of ids attached to an
element. This would also match situation at XPath identity constraints 
where it is
also allowed to define an arbitrary number of key and unique constraints.

Thanks for your attention,
--Stefan