Reserved namespaces, and QNames as extension keywords

[Pavel Minaev]

I have two questions regarding implementation conformance to the XQuery spec.

1) The spec requires every module and schema to be associated with a
namespace, and seemingly doesn't restrict the value domain for such
namespaces (except for restricting clashes between several
modules/schemas imported within a single query). Can an XQuery
implementation go further than that, and reserve specific namespaces
for its own use, not allowing programmer to define his modules and
schemas within that namespace at all? (but, e.g., providing some
additional predefined types available from such namespaces without any
"import schema" declarations).

2) Assuming the answer to #1 is true, is it legal for a conforming
XQuery implementation to use QNames from such a reserved namespace as
"keywords" (i.e. syntax elements for extension language constructs),
without fear of clashing with future XQuery versions? I.e. something
along the lines of:

  declare namespace ext = "...";

  declare variable $f
  as ext:function(xs:integer, xs:integer => xs:integer) (* custom type
keyword *)
  := ext:lambda($x as xs:integer, $y as xs:integer) as xs:integer { $x
+ $y } (* custom syntactic construct *)

I am aware of pragmas and "declare option", but, obviously, they are
much more limited - in example above, ext:lambda could possibly be a
pragma, though that would complicate nesting; but I don't see any way
to use extensibility mechanisms explicitly spelled out in XQuery spec
for something like ext:function.

If the existing spec explicitly disallows such things, then I would
like to propose enabling them in future XQuery versions; maybe with a
new special construct to declare such "extensibility" namespaces for
extra clarity, e.g.:

  declare extension namespace ext = "...";