This command takes one argument, the namespace URI,
possibly accompanied by a prefix provided in
both these expressions are evaluated as names.
The command changes the namespace of the current element to a given
namespace URI. The current node must be in the scope
of a namespace declaration associating the namespace URI
with a prefix; if prefix option is given, then
one of such declarations must associate the particular given prefix
with the namespace URI. If this condition
is not met or the current node is neither element nor
attribute, an error is issued.
The command also changes the prefix of the current element accordingly.