docs: def set_source(self, source_id=None):
          """Sets the source.
  
          :param source_id: the new publisher
          :type source_id: ``osid.id.Id``
          :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.*
  
          """
          if source_id is None:
              raise NullArgument()
          metadata = Metadata(**settings.METADATA['source_id'])
          if metadata.is_read_only():
              raise NoAccess()
          if self._is_valid_input(source_id, metadata, array=False):
              self._my_map['sourceId'] = str(source_id)
          else:
              raise InvalidArgument()