code: def find_causal_link(self, direction, mechanism, purviews=False,
                           allow_neg=False):
          
          purviews = self.potential_purviews(direction, mechanism, purviews)
  
          # Find the maximal RIA over the remaining purviews.
          if not purviews:
              max_ria = _null_ac_ria(self.mechanism_state(direction),
                                     direction, mechanism, None)
          else:
              # This max should be most positive
              max_ria = max(self.find_mip(direction, mechanism, purview,
                                          allow_neg)
                            for purview in purviews)
  
          # Construct the corresponding CausalLink
          return CausalLink(max_ria)
docs: Return the maximally irreducible cause or effect ratio for a
          mechanism.
  
          Args:
              direction (str): The temporal direction, specifying cause or
                  effect.
              mechanism (tuple[int]): The mechanism to be tested for
                  irreducibility.
  
          Keyword Args:
              purviews (tuple[int]): Optionally restrict the possible purviews
                  to a subset of the subsystem. This may be useful for _e.g._
                  finding only concepts that are "about" a certain subset of
                  nodes.
  
          Returns:
              CausalLink: The maximally-irreducible actual cause or effect.