def set_source(self, source_id=None):
pass<EOL>
Sets the source.

        arg:    source_id (osid.id.Id): the new publisher
        raise:  InvalidArgument - ``source_id`` is invalid
        raise:  NoAccess - ``Metadata.isReadOnly()`` is ``true``
        raise:  NullArgument - ``source_id`` is ``null``
        *compliance: mandatory -- This method must be implemented.*