RE: [xml-dev] Re: determining ID-ness in XML

["Champion, Mike" <Mike.Champion@SoftwareAG-USA.com>]

> 
>  xml:id and/or xml:idatts 
> can be layered on top of XML 1.0 without changing anything 
> about XML 1.0. 

What does "without changing anything about XML 1.0" mean, exactly? Could the
xml+id processor be actually layered on top (or in front of) an  XML 1.0
processor without changing the DTD validation logic?  Does it mean that
valid and  well-formed XML 1.0 documents will continue to be so in xml+ids?
How about schema processors; will an "old" schema processor interoperate
with an xml+ids XML processor? 

One question (brought up by Michael Kay in an internal discussion): "is
xml+id"  non-uniqueness a well-formedness error, or will it be some sort of
low-level validation error? Presumably a well-formedness error, since
non-validating parsers have no notion of "validation". BUT if there is a
schema-defined ID and a XML+id-defined ID that clash, it is presumaby a
validity error, because schemas only apply to well-formed XML documents ...
andy anyway this implies that the schema processor knows about xml+id.  

These potential complications seem to suggest that doing any more than the
bare minimum could start to get hairy. I'm inclined against adding ISO
entities, multiple ids/element, namespace support, etc. in the first
iteration.