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.

def AddDatastore(self, urn):
    """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.
    """
    if urn not in self._datastores:
      self._datastores.add(urn)
      return True
    return False