Declare a namespace for use by the query. This has the same
status as a namespace appearing within the query prolog (though
a declaration in the query prolog of the same prefix will take
precedence)
The namespace prefix to be declared. Use
a zero-length string to declare the default namespace (that is, the
default namespace for elements and types).
uri
The namespace URI. It is possible to specify
a zero-length string to "undeclare" a namespace.