code: def get_transition_chempots(self, element):
          if element not in self.elements:
              raise ValueError("get_transition_chempots can only be called with "
                               "elements in the phase diagram.")
          critical_chempots = []
          for facet in self.facets:
              chempots = self._get_facet_chempots(facet)
              critical_chempots.append(chempots[element])
          clean_pots = []
          for c in sorted(critical_chempots):
              if len(clean_pots) == 0:
                  clean_pots.append(c)
              else:
                  if abs(c - clean_pots[-1]) > PhaseDiagram.numerical_tol:
                      clean_pots.append(c)
          clean_pots.reverse()
          return tuple(clean_pots)