code: def AddDatastore(self, urn):
      
      if urn not in self._datastores:
        self._datastores.add(urn)
        return True
      return False
docs: Adds a datastore URN as a source.
  
      Args:
        urn: an RDF URN value of the datastore.
  
      Returns:
        True if the datastore is not an already existing source.