p:namespaces proposals

Currently, we say it's a static error (err:XS0051) if the value of the
exclude-prefixes attribute is wrong. I've changed that into a dynamic
error because you can't check the prefixes until runtime.

We also say:

  It is a dynamic error (err:XD0013) if the specified
  namespace bindings are inconsistent; that is, if the same prefix is
  bound to two different namespace names.

Do we really want that? Does it make more sense to construct the
mapping in the order in which the p:namespace elements are specified
and just say that if the same prefix is defined multiple times, the
last binding wins?

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Great men too make mistakes, and many
http://nwalsh.com/            | among them do it so often that one is
                              | almost tempted to call them little
                              | men.-- Lichtenberg