code: def potential_purviews(self, direction, mechanism):
          
          all_purviews = utils.powerset(self._node_indices)
          return irreducible_purviews(self.cm, direction, mechanism,
                                      all_purviews)
docs: All purviews which are not clearly reducible for mechanism.
  
          Args:
              direction (Direction): |CAUSE| or |EFFECT|.
              mechanism (tuple[int]): The mechanism which all purviews are
                  checked for reducibility over.
  
          Returns:
              list[tuple[int]]: All purviews which are irreducible over
              ``mechanism``.