Can we treat XML elements and attributes as sets

I am reading Elliotte Rusty Harold's excellent book
"XML in a Nutshell". I have question about XML
Namespaces.

Elliotte mentions in the chapter about XML Namespaces,
that "The URIs partitions the elements and attributes
into disjoint sets".

I don't mean to say this wrong. But is this statement
really correct?

Regards,
Mukul

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