Is a namespace a set?

[Klaus Backert <KlausBackert@compuserve.com>]

Hi,

several contributors to this list said, that a namespace is a set.

In http://www.w3.org/TR/1999/REC-xml-names-19990114 the first definition
states:
[Definition:] An XML namespace is a collection of names ... XML namespaces
differ from the "namespaces" conventionally used in computing disciplines in
that the XML version has internal structure and is not, mathematically
speaking, a set ...

I would appreciate an explanation very much, because I want to learn more
about namespaces - and not only to follow the rather confusing RDDL-thread
;-) Is a namespace a set or not? Does it depend? Does it matter?

Klaus