def get_identifier_namespace(self):
return self._namespace<EOL>
Gets the namespace of the identifier.

        The namespace reflects the domain in which the identifier is
        unique. When using a global identifier schema, the namespace may
        indicate the name of the scheme. When using a local
        identification scheme, the namespace may be more specific, such
        as the name of a database or file in which the identifiers
        exist. Federating adapters may use a custom namespace to include
        information for routing. This method is used to compare one
        ``Id`` to another.

        return: (string) - the authority of this ``Id``
        *compliance: mandatory -- This method must be implemented.*