Get the critical chemical potentials for an element in the Phase
        Diagram.

        Args:
            element: An element. Has to be in the PD in the first place.

        Returns:
            A sorted sequence of critical chemical potentials, from less
            negative to more negative.

def get_transition_chempots(self, element):
        """
        Get the critical chemical potentials for an element in the Phase
        Diagram.

        Args:
            element: An element. Has to be in the PD in the first place.

        Returns:
            A sorted sequence of critical chemical potentials, from less
            negative to more negative.
        """
        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)